docs(a2a): fix chat client profile URL

[FanouZeng-TT]

Description

a2a/README.md points readers to the Chat Client UCP Profile at:

http://localhost:3000/profile/agent-profile.json

The checked-in profile is named chat-client/profile/agent_profile.json, and the frontend plus the testing guide both use the underscore path. With Vite running, the documented hyphenated URL falls back to the SPA HTML response instead of returning the profile JSON.

Fix: update the README URL to /profile/agent_profile.json, matching the static file and the client request header.

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)

Related Issues

N/A

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)

Verified locally with the Vite dev server: /profile/agent_profile.json returns HTTP 200 with Content-Type: application/json and parses as the checked-in UCP profile. The chat client production build and the repository-wide pre-commit suite also pass.