Add isolated CI coverage for extras and guard optional-dependency tests

[ftnext]

Summary

• split unit test CI into a base job, an all-extras regression job, and per-extra contract checks
• verify each extra in a fresh environment so missing transitive dependencies are not masked by other extras
• make optional-dependency tests skip cleanly when their extra is not installed

Background

This follows the dependency mismatch discussed in #855 and fixed in #856.

That issue was easy to miss because CI mainly exercised an all-extras environment. In that setup, dependencies installed by one extra can mask missing dependencies in another extra. In the faster-whisper case, soundfile could be present because of other installs, so the faster-whisper packaging contract was not being verified in isolation.

What This Changes

• add a base job that installs only .[dev]
• keep the existing all-extras regression coverage
• add extra-contracts jobs that create a fresh environment per extra and run targeted verification
• update optional-dependency tests to skip cleanly when their extra is not installed

Testing

• git diff --check
• python3 -m compileall tests speech_recognition

A full local pytest run in a fresh CI-like environment was not possible from this worktree, so the new workflow is intended to validate the matrix in CI.

[ftnext]

codex

The changes add CI coverage for base and per-extra installations and make optional-dependency tests skip when their corresponding extras are not installed.
I did not find any discrete issues in the diff that would break existing tests or workflows.