DAOS-17321 ddb: Add checksum check function to ddb vos API

[knard38]

This patch is part of a 3-way split of #18940 ("DAOS-17321 ddb: add csum_check command to verify data checksums"), which was too large to review as a single PR. The split mirrors the layering already used for the sibling csum_dump feature earlier in this same ticket (#18444 -> #18543 -> #18685):

1. This patch — adds dv_check_csum() to the ddb VOS API (ddb_vos.c/ddb_vos.h), with unit tests.
2. patch-007 (stacked on this one) — wires ddb_run_csum_check() into the ddb C command set on top of dv_check_csum(). More details could be found in the draft PR DAOS-17321 ddb: Add checksum check command to ddb C API #18942.
3. patch-008 (stacked on patch-007) — wires csum_check through to the ddb Go CLI, plus documentation. More details could be found in the draft PR DAOS-17321 ddb: Add checksum check command to ddb GO code #18943.

Supersedes #18940, which contained all three layers as a single squashed commit.

Changes

dv_check_csum() — new ddb VOS API function (ddb_vos.c, ddb_vos.h)

dv_check_csum(daos_handle_t poh, struct dv_tree_path *path, daos_epoch_t epoch, dv_check_csum_cb check_cb, void *cb_arg) mirrors the structure of the existing dv_dump_csum() (open container, build the iod from path, dispatch by value type, close container), but additionally fetches the live data for each checksummed segment, recomputes its checksum(s) with a throwaway daos_csummer, and compares the result against what was already fetched from disk. Nothing is modified — this is a read-only verification pass.

Results are reported through a callback:

typedef int (*dv_check_csum_cb)(void *cb_arg, struct daos_recx_ep_list *recx_rel,
                                daos_epoch_t sv_epoch, struct dcs_ci_list *cil,
                                struct dcs_csum_info **got_csums);

got_csums is an array of cil->dcl_csum_infos_nr pointers: got_csums[i] holds the checksum recomputed from the currently stored data when entry i's stored checksum did not match, and is NULL when it matched (or there was nothing to check) — so got_csums[i] != NULL doubles as the per-entry mismatch flag. The function itself returns -DER_CSUM if any entry failed to match, after invoking the callback with the full picture (so callers can report every mismatch, not just the first one).

Internally, check_csum_sv()/check_csum_recx() fetch the checksum metadata via the same VOS_OF_FETCH_CSUM mechanism as the dump path, then verify_segment_csum() fetches the live data for that segment (fetch_check_value()), recomputes its checksum, and compares it (daos_csummer_compare_csum_info()), snapshotting a standalone copy of the recomputed checksum (snapshot_csum_info()) on a mismatch so it outlives the throwaway csummer. For array akeys, all overlapping segments are checked independently — a corrupted segment doesn't stop verification of the others, and got_csums[] correctly isolates which segment(s) failed.

As a drive-by cleanup in the same file, dump_csum_sv()/dump_csum_recx() had their cb_rc/rc double-bookkeeping simplified (no behavior change).

Test fixtures (tests/ddb_test_driver.c, tests/ddb_test_driver.h)

Adds a third checksum test object (g_oids[2]) with valid data but a deliberately corrupted stored checksum (single-byte flip applied to an otherwise-correct checksum before writing), for both the single-value and array-akey layouts:

• csum_test_corrupt_sv_setup() — one corrupted SV.
• csum_test_corrupt_recx_setup() — an array with DVT_FAKE_RECX_COUNT overlapping segments, only DVT_FAKE_RECX_BAD_IDX of which is corrupted, so per-entry isolation can be exercised (the other segment(s) must never be flagged).

Tests (tests/ddb_vos_tests.c)

Three new test cases under the existing csum test suite:

Test	What it covers
check_csum_error_tests	Invalid pool handle -> vos_cont_open failure propagation
check_csum_sv_tests	SV path: no-csum (success, nothing to check), matching csum at a specific epoch and at EPOCH_MAX, corrupted csum (-DER_CSUM, got_csums[0] holds the recomputed value), no-callback no-op, callback-error propagation
check_csum_recx_tests	Array path: no-csum, matching csum (multiple overlapping segments), corrupted csum with per-entry isolation (only the deliberately-corrupted segment is flagged), no-callback no-op, callback-error propagation

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'Checksum management with ddb'
Status is 'In Progress'
https://daosio.atlassian.net/browse/DAOS-17321

[daosbuild3]

Test stage Functional Cluster Box Medium Verbs Provider MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18941/1/execution/node/1599/log

[daosbuild3]

Test stage Functional Cluster Box Medium MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18941/1/execution/node/1588/log

[daosbuild3]

Test stage Functional Hardware Large MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18941/2/execution/node/1724/log

[daosbuild3]

Test stage Functional Hardware Medium Verbs Provider MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18941/2/execution/node/1821/log

[daosbuild3]

Test stage Functional Hardware Medium MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18941/2/execution/node/1874/log