docs(rest/python): fix UCP-Agent curl quoting

[FanouZeng-TT]

Description

Two curl examples in rest/python/server/README.md nested unescaped double
quotes around the UCP-Agent header:

-H "UCP-Agent: profile="https://agent.example/profile"" \

The shell closes the outer string before the profile URL, so copying the command
does not pass the documented header value as one argument. The Python client's
existing happy_path_dialog.md example already uses the correct quoting form.

Fix: wrap each full header argument in single quotes while preserving the
double-quoted profile URL.

Category (Required)

• Core Protocol: Changes to the core UCP protocol. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance or contributing processes. (Requires Governance Council approval)
• Capability: New or updated UCP capabilities. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: Build, CI, and repository infrastructure. (Requires DevOps Maintainer approval)
• Maintenance: Dependency and repository maintenance. (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. (Requires Maintainer approval)
• Community Health (.github): Organization-wide community files. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide and the project's style guidelines.
• 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)

• Shell argument parsing confirms the corrected header remains a single -H value.
• pre-commit run --all-files: passed.
• git diff --check: passed.