Add --reruns-on-exitfirst to override rerun count under -x/--maxfail

[LouisDeconinck]

Summary

• Adds --reruns-on-exitfirst N: when -x/--exitfirst or --maxfail is given, the rerun count for every test becomes N instead of the global/marker setting
• --reruns-on-exitfirst 0 makes -x exit on the first failure immediately, without waiting for reruns
• --force-reruns still wins; negative values are rejected with a UsageError

Closes #249

Test plan

• -x --reruns-on-exitfirst 0 disables global and marker reruns
• Non-zero value limits reruns under -x
• Applies under --maxfail too; behavior unchanged without the option; negative value rejected

[sylvesterkaczmarek]

--reruns-on-exitfirst can enable reruns under --pdb without tripping the existing incompatibility check. check_options() only considers --force-reruns / the global rerun setting, while get_reruns_count() now returns this new value whenever maxfail is active. Thus pytest -x --pdb --reruns-on-exitfirst 1 is accepted and will rerun failures even though the plugin otherwise rejects reruns with PDB. Please include the new option in the --pdb validation and add that no-global-reruns case to the tests.

[LouisDeconinck]

Fixed in 7da4ffb: check_options() now treats --reruns-on-exitfirst as enabling reruns under -x/--maxfail when no other rerun count is set, so pytest -x --pdb --reruns-on-exitfirst 1 is rejected. Added regression tests for the error and the 0 opt-out case.

[LouisDeconinck]

Fixed — check_options() now rejects --reruns-on-exitfirst under --pdb alongside the other rerun options, with tests covering both the rejected case and the zero-value opt-out (7da4ffb).