Raise traverse-mcp line coverage from 86% to 98.55%

[enricopiovesan]

Summary

• Quality: Enforce coverage gate for traverse-cli and traverse-mcp (constitution 100% core logic) #594 added traverse-mcp to the coverage gate with a measured phased floor (86.07%) so it couldn't regress while a larger test tranche was completed. This closes most of that gap.
• lib.rs 74.6%→99.9%: not_found/map_runtime_error/lifecycle_name exercised directly and exhaustively (every RuntimeErrorCode and Lifecycle variant), McpToolRegistry Default/Debug, observe_execution, and get_capability/get_event/get_workflow's not-found paths.
• stdio_server.rs 74.6%→94.2%: StdioAuthConfig's Debug impl and from_env (refactored into an injectable from_env_var core, since mutating env vars from a test is unsafe and this workspace forbids unsafe entirely); describe_entrypoint/describe_content_group/entrypoint_artifacts/validate_runtime_request's validation branches driven directly or via crafted stdio commands; run_stdio_server's own body via simulate_startup_failure (which returns before ever reading real stdin, so it's safe to call directly); every write_json_line call site's io_error branch, exercised with a writer that fails on the Nth \n-terminated envelope so each branch can be targeted individually; malformed-JSON and invalid-UTF8 command lines; unsupported commands; and several free helper functions called directly.
• main.rs 0%→75%: extracted a testable run(args) core from main, covering the usage/unsupported-command/simulated-failure branches.
• Converted several test-only Err(e) => panic!(...) match arms into single-line .expect()/.expect_err() calls: those arms are unreachable in a passing test and llvm-cov's line coverage counts the never-taken arm as a separate missed line.
• ci/coverage-targets.txt's traverse-mcp threshold moves from 86 to 98 (measured 98.55%, kept a small margin for tooling variance, matching the convention of the original phased floors).
• Remaining gap (98.55%→100%) is registry-bundle-load failure paths with no test injection point, one genuinely unreachable executor match arm, and a get_capability double-lookup branch that doesn't appear reachable through the public registry API — filed as #622 per the Definition of Done.

Governing Spec

• 015-capability-discovery-mcp
• 042-mcp-library-surface

Project Item

Closes #617

Validation

• cargo test --workspace — all tests pass (47 new/updated traverse-mcp tests).
• cargo clippy --workspace --all-targets -- -D warnings — clean.
• cargo fmt --check — clean.
• bash scripts/ci/coverage_gate.sh (with LLVM_COV/LLVM_PROFDATA pointed at brew llvm) — passes; traverse-mcp measured at 98.55%, threshold set to 98.
• bash scripts/ci/spec_alignment_check.sh — passes locally against this branch.