ucp version data format correction in samples

[dipankar1415]

Description

Fix UCP version in the Python REST merchant sample (rest/python/server).

validate_ucp_headers previously compared agent and merchant versions with string ordering (agent_version > server_version), which is incorrect for UCP YYYY-MM-DD versions. In discovery_profile.json looks like it is in date format, for example:
"version": "2026-01-23"

This change parses versions as ISO calendar dates, compares datetime.date values, returns VERSION_INVALID_FORMAT (400) for malformed agent versions, and returns VERSION_UNSUPPORTED (400) when the agent version is newer than the merchant. Behavior is unchanged when UCP-Agent omits version= (defaults to merchant version).

Files changed: ucp_version.py (new), exceptions.py (UcpVersionError), dependencies.py (date-based validation).
Note: the tests are not added, can add the tests if instructed to add them at a new tests folder.

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)

Example response for a newer agent version (version="2027-01-01" vs merchant 2026-01-23):

{
"detail": {
"status": "error",
"errors": [{
"code": "VERSION_UNSUPPORTED",
"message": "Version 2027-01-01 is not supported. This merchant implements version 2026-01-23.",
"severity": "critical"
}]
}
}

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