why

the protocol now lets the worker ask an SDK to make an HTTP request. the python SDK needs an implementation that can receive that request, send it over HTTP, & return the response while the original Stagehand call is still waiting

this PR adds the python implementation of #3205 using the SDK's existing httpx dependency & RPC connection

what changed

• added a connection snapshot with the same browser-factory precedence, regional endpoints, & API URL override behavior as the TypeScript implementation
• added one HTTP attempt per Gateway, cache read, or cache write request. the executor applies bb headers & returns the buffered status, headers, & body, including HTTP errors & redirects.
• used the caller's remaining timeout through body buffering, without adding an httpx default timeout or a second retry loop
• added request & cancellation handlers to the existing RPC connection. explicit request cancellation, task cancellation, disconnect, & disposal stop the corresponding HTTP work & release its resources

test plan

• shared fixtures & connection tests cover endpoint selection, precedence, & snapshots remaining unchanged after caller inputs are mutated.
• local HTTP tests cover header precedence, response decoding, redirects, network failures, & deadlines before headers and during body delivery.
• RPC tests cover requests during a pending batch call, cancellation, duplicate IDs, independent connections, & handler disposal

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Summary by cubic

Adds the Python SDK implementation of the Browserbase HTTP transport, letting a worker ask the SDK to make HTTP requests over the RPC connection while the original Stagehand call is still pending.

• Resolves a Browserbase connection snapshot (API key, API URL, session ID) with the same precedence, regional endpoints, and override behavior as the TypeScript SDK.
• Executes one httpx request per Gateway, cache read, or cache write, returning the buffered status, headers, and body — including HTTP errors and redirects — with no retry loop or httpx default timeout.
• Uses the caller's remaining timeout through body buffering, without adding a second deadline.
• Adds request and cancellation handlers to the RPC connection; cancellation, task cancellation, disconnect, and disposal stop in-flight HTTP work and release resources.