fix: give the attach path the same 10 minutes as session creation

[ajma]

What

_wait_for_session_available now defaults to a 600s budget instead of 300s.

Why

getOrCreate has two paths that each wait for a session to become usable, and their budgets had drifted apart:

• Create (__create) polls the long-running operation returned by create_session, with timeout=600 on the polling retry.
• Attach (_get_exiting_active_session) has no operation handle to poll, so it hand-rolls a GetSession loop waiting for the "Spark Connect Server" key to appear in runtime_info.endpoints — and that loop allowed only 300s.

Both are waiting on the same underlying thing: a server finishing its boot. Because get_active_s8s_session_response treats CREATING as reusable, the attach path can land on a session that hasn't started yet and then give up at half the budget the create path would have allowed. The most common way to hit this is passing a custom session ID that resolves to a session someone else just started.

Raising the default to 600s means a caller gets the same 10 minutes either way.

Not addressed here

The loop only evaluates its deadline between iterations, and get_session is issued with no per-call timeout (the Dataproc GAPIC sets default_timeout=None for every method). So the wall-clock can still overshoot the stated budget by the duration of a stalled final call — it will now read "after 600 seconds" while taking longer. Bounding that properly means threading the remaining budget into the get_session call; left for a separate change. _wait_for_termination has the same shape with a 180s budget.

Testing

pytest tests/unit/test_session.py -k wait_for_session_available passes (both tests pass an explicit timeout, so neither depended on the default). pyink --check clean.

[gemini-code-assist]

Code Review

This pull request increases the default timeout in the _wait_for_session_available method from 300 to 600 seconds in google/cloud/managed_spark_connect/session.py. There are no review comments, and I have no feedback to provide.