Document per-request HTTP headers for Streamable HTTP clients

[HaoChiBao]

Summary

• Documents the existing Client + Streamable HTTP pattern for per-request Authorization / trace headers using contextvars and an httpx2 event_hooks["request"] hook on a shared AsyncClient (docs_src/client_transports/tutorial005.py + section in docs/client/transports.md).
• No new Client kwargs or public API; this is the docs-only path @Kludex suggested on Per-Request Transport Configuration for MCP Clients #1966.
• Adds a docs_src test that proves values set around each Client call reach the request hook as headers over in-process Streamable HTTP.

Fixes #1966

AI disclosure

AI-assisted drafting (Cursor). I reviewed the resulting docs/example/tests and verified them locally.

Test plan

• uv run --frozen ruff format / ruff check --fix on touched files
• uv run --frozen pyright on touched files
• uv run --frozen pytest tests/docs_src/test_client_transports.py -v (7 passed)
• Confirm docs page renders the new --8<-- snippet under Client transports → Per-request headers

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[github-actions]

Thanks for the contribution. This repository only keeps pull requests open when they're linked to an issue that a maintainer has assigned to the author — CONTRIBUTING.md explains why and how we work. This PR has been closed for now because you aren't currently assigned to #1966.

If a maintainer would like this change as a PR from you, they'll assign you to #1966 and this PR will reopen automatically — there's nothing more you need to do. (If you opened the issue, this PR already shows up on its timeline.)

There's no need to open a new PR — this one will be reopened. While it's closed, please push any updates as new commits rather than force-pushing, since GitHub can't reopen a PR whose branch has been rewritten.

Maintainers: reopening this PR, removing the missing-issue-link label, or adding bypass-issue-check bypasses the check.