feat: one exception family for an invalid ttl (#337)

[allen0099]

Fixes #337

Summary

An invalid ttl raised CacheXError in @cache, but TypeError or ValueError in the backends, CacheManager and CacheLock. Code validating a user-supplied TTL had to catch all three.

This PR follows the issue's proposal: a CacheXError subclass that keeps the built-in bases, so no existing except clause breaks.

CacheXError
└── InvalidTTLError          catch this for any invalid ttl
    ├── TTLTypeError(InvalidTTLError, TypeError)
    └── TTLValueError(InvalidTTLError, ValueError)

These now raise the new classes:

• validate_ttl(), and therefore every backend, CacheManager, CacheLock and @cached
• @cache's check on ttl and stale_ttl
• the Memcached check for an expiry after 2038-01-19

Old except clauses keep working:

• except TypeError and except ValueError around backend, manager or lock calls still catch.
• except CacheXError around @cache still catches.
• The only new behaviour: an except ValueError around @cache(ttl=-1) now also catches. Nothing changes otherwise.

Other changes:

• All three names are exported from fastapi_cachex and documented by the API reference (::: fastapi_cachex.exceptions).
• "TTL values" in BACKENDS.md and the ttl=0 note in HTTP_CACHING.md are updated in English and zh-TW, with an except InvalidTTLError example.
• validate_delta (the counter delta, not a TTL) is unchanged.

The docstrings and docs say "before 0.5.1", assuming this ships in the next patch release. If you release it as 0.6.0, they need changing.

Checklist

• The issue above is assigned to me (maintainer's own issue)
• Tests under tests/ cover the change. tests/test_ttl_errors.py checks each entry point (backend set, CacheManager and its set, CacheLock, @cached, @cache ttl/stale_ttl, the Memcached 2038 check) for the exact class and both bases. Reverting one raise to TypeError fails 12 tests, and reverting @cache's to CacheXError fails 2.
• A changelog fragment: changelog.d/337.added.md
• uv run pre-commit run --all-files and uv run pytest --cov pass: 1243 passed, 192 skipped, 92.90% coverage. So do mypy strict, mypy tests, mypy scripts and both zensical build --strict runs.

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