fix: evaluate extra == markers per-extra with OR semantics

[xangcastle]

_decide_marker_impl was joining multiple extras into a single comma-separated string before passing it to the PEP-508 evaluator, so extras = ["dev", "other"] became extra = "dev,other" which never matched a marker like extra == 'dev', causing optional dependencies gated by extras to be silently excluded.
This PR fixes that by evaluating the marker once per extra and ORing the results, so a dependency is included if any of the active extras matches. It also adds venv-derived extras inference: since generated decide_marker calls never pass extras explicitly, the fix now reads the active venv name from the _venv flag and extracts matching extra values from the marker string, which handles conflict group markers of the form extra == 'group-N-project-venvname'.
Tests added to pep508_evaluate_test.bzl cover positive/negative extra == cases including the comma-join regression, and new decide_marker targets in BUILD.bazel exercise the OR evaluation path directly.

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Test plan

• New test cases added

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[jbedard]

Please add the same tests (and possibly fixes) to main if applicable.