fix: validate capability version dates

[horaceewang-TT]

Description

Fix capability version validation to prevent malformed or impossible date strings from bypassing requires compatibility checks.

Validate capability versions during parsing.
Reject invalid actual versions before lexicographical comparison.
Validate Gregorian calendar dates, including month lengths and leap years.
Add regression tests for malformed versions, invalid dates, and leap-year boundaries.

Category (Required)

Please select one or more categories that apply to this change.

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

$ cargo test --all-targets
test result: ok. 154 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
test result: ok. 78 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
test result: ok. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
test result: ok. 68 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Unit tests covering this fix:

1. types::tests::is_valid_version_format

   • valid versions: 2026-01-23, 2025-12-31
   • invalid formats: 2026-1-23, not-a-date, 20260123, empty string
   • invalid calendar dates: 2026-13-32, 2026-00-15, 2026-06-00, 9999-99-99
   • invalid month/day combinations: 2026-02-29, 2026-02-31, 2026-04-31
   • leap-year handling: accepts 2024-02-29, accepts 2000-02-29, rejects 1900-02-29

2. types::tests::version_constraint_satisfied_by

   • rejects versions below min
   • accepts min boundary inclusively
   • accepts versions within range
   • accepts max boundary inclusively
   • rejects versions above max
   • rejects invalid actual versions: not-a-date, 2026-02-31

3. compose::tests::parse_capabilities_rejects_invalid_version

   • rejects a capability entry whose version is not-a-date
   • returns ComposeError::InvalidCapability