fix(rest/python): serve the discovery profile with a Cache-Control header

[vishkaty]

Description

The discovery profile at /.well-known/ucp is returned with no Cache-Control header. The spec requires profile responses to be cacheable (overview.md, Discovery: profile caching):

Profile responses MUST include a Cache-Control header with public and max-age of at least 60 seconds. Profiles MUST NOT be served with private, no-store, or no-cache directives.

This PR sets Cache-Control: public, max-age=3600 on the profile response and adds an integration test asserting the header is present, public, carries a max-age >= 60, and uses none of the forbidden directives.

Verified locally: the new test fails on the current handler and passes with the fix; the full rest/python/server integration suite stays green; and a booted server returns the header on the wire.

Category (Required)

• 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)

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.

[damaz91]

Thanks!