test(recorded): add rails library coverage (5/5)

[Pouyanpi]

Summary

Adds recorded coverage for library rails and their composition order.

Why

Built-in rails need replayable end-to-end coverage so future rail changes can be reviewed against stable behavior.

What Changed

• Adds library rail configs, cassettes, and tests for regex, injection, self-check, content safety, jailbreak, topic control, and composition.
• Adds library README guidance requiring recorded coverage for new rails.

Review Notes

This is the final stack PR and includes no additional runtime changes.

Stack Position

Part 5 of 5.

• Previous: test(recorded): add rails public API coverage (4/5) #1977
• Next: none, final stack PR

Stack Context

This stack decomposes recorded end-to-end replay coverage into reviewable slices. The PRs should be reviewed against their parent branch in the stack.

Please review each PR against its parent branch, not directly against the root base branch, except for part 1.

Order	PR	Branch	Base
1	#1974	stack/recorded-tests-01-harness	develop
2	#1975	stack/recorded-tests-02-deterministic-library-load	stack/recorded-tests-01-harness
3	#1976	stack/recorded-tests-03-clients	stack/recorded-tests-02-deterministic-library-load
4	#1977	stack/recorded-tests-04-public-api	stack/recorded-tests-03-clients
5	#1978	stack/recorded-tests-05-library-rails	stack/recorded-tests-04-public-api

Validation

poetry check --lock
poetry lock --no-update
poetry install --with dev
poetry run pytest tests/recorded --block-network -q
pre-commit hooks passed during commit creation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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