sdk/python: bound a claim by the caller's deadline

[CMGS]

Closes #182.

Client._request gave every control-plane request a fresh client.timeout, and the redirect walk on a warm miss is a loop of such requests. The LangChain toolkit promises a five-minute wall clock per sandbox_exec and, since #179, honours it for the run — but not for the claim: two black-holed redirect candidates cost 600 s with a 300 s client timeout, so the tool could exceed its promise on a cluster with unreachable peers.

Claims now take an absolute time.monotonic() deadline:

• Client.new, Template.new and Checkpoint.new accept keyword-only deadline, threaded through _claim_from, _post_json and the redirect walk into _request
• _request bounds each attempt by min(timeout, deadline - now) and raises TimeoutError once the deadline is spent, before the request is built — _try_each and _redirect_fallback only catch APIError, so the walk stops there instead of trying the next candidate
• the deadline arithmetic is the one conn.remaining_timeout the dial path already used, now shared rather than duplicated (it gained the operation name for the error message)
• CocoonToolkit._exec passes the deadline it already computes, through sandbox() into _claim

Measured against a node that redirects to two addresses which accept TCP and never answer, client timeout 2 s:

no deadline (old behavior): APIError after 4.01s
deadline +0.3s:            TimeoutError after 0.30s

Default behavior is unchanged: without a deadline every request still gets the full timeout.

Tests: test_claim_refuses_a_spent_deadline (the node sees no request at all), test_claim_deadline_bounds_the_redirect_walk (two black holes, fails in 0.3 s instead of 2 x 2 s), plus test_exec_claims_inside_the_call_budget and test_exec_reports_a_claim_that_outlives_the_budget on the toolkit. conftest gained a black_hole fixture — a listening socket that never accepts.

Left alone deliberately: Client.lookup scatters its probes concurrently under min(_PEERS_TIMEOUT, timeout) = 5 s, so it cannot run away the same way, and the OpenAI adapter makes no wall-clock promise over its claim (its timeout is caller-supplied and applies to run).

Gates: ruff format --check and ruff check clean in sdk/python, sdk/openai and sdk/langchain; pytest 195 passed in sdk/python and 8 in sdk/langchain. sdk/openai's suite does not collect on this machine (openai-agents is not installed); it is untouched by this change and CI runs it.