Add comprehensive tests for provider plugin system

[aksg87]

Description

Adds comprehensive test coverage for the provider plugin system introduced in #97.

Related to #99, #111

Feature (Testing)

Changes

• Smoke tests for plugin discovery, registration, and usage flows
• Edge case handling (malformed plugins, import errors)
• Plugin priority override validation
• Environment variable controls (LANGEXTRACT_DISABLE_PLUGINS)
• E2E test simulating pip-installed plugin lifecycle
• CI/CD integration with new tox environments

How Has This Been Tested?

pytest tests/provider_plugin_test.py -v
# 8 passed

tox -e plugin-smoke
tox -e plugin-integration

Checklist:

• I have read and acknowledged Google's Open Source Code of conduct.
• I have read the Contributing page, and I either signed the Google Individual CLA or am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed Google's Python Style Guide and ran pylint over the affected code.

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[github-actions]

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[github-actions]

Manual Validation Results

Status: ❌ Failed

Check	Status	Details
PR Size	✅	504 lines
Template	❌	Missing required sections
Linked Issue	❌	Missing Fixes/Closes #XXX

Errors:

• ❌ Missing PR template sections: Fixes #
• ❌ No linked issue found

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