fix: Keep a stream open for as long as it runs, rather than end it after five minutes.

[goloroden]

HttpClient created its aiohttp.ClientSession without a timeout, so aiohttp used its default ClientTimeout(total=300, sock_connect=30), and total also covers reading the response body. Every observe_events stream therefore ended after exactly five minutes with a bare TimeoutError, even while the server sent a heartbeat every second, and so did any other stream that legitimately ran longer (a long run_eventql_query, read_events over a big store).

• The five calls that read a stream of NDJSON lines (read_events, run_eventql_query, observe_events, read_subjects, read_event_types) pass is_stream=True to HttpClient.post, which gives that request ClientTimeout(total=None, sock_connect=30): no limit on how long it runs, while connecting still fails after 30 seconds. How long a stream may run is up to the caller (for example with asyncio.timeout); a stalled observe stream or query ends after 30 seconds without a line through the heartbeat timeout (feat: End a stream with an error when neither an event nor a heartbeat arrives for 30 seconds. #268).
• Every other request (ping, write_events, verify_api_token, register_event_schema, read_event_type) keeps the session's limit, unchanged. It is per request because aiohttp reads a per-request timeout=None as no limit at all, connecting included. The public Client API is unchanged; HttpClient.post only gains an optional argument.
• README: one sentence at the top of "Aborting Observing".
• The local test server from feat: End a stream with an error when neither an event nor a heartbeat arrives for 30 seconds. #268 now answers every POST path, so the new tests use it, too.

Tests (with aiohttp's default patched to a 1 s total): observing against the real EventSourcingDB container stays open for the test's full 3 s deadline; a query against the local server sending heartbeats does the same; a write to a server that never sends its body still ends through the session limit, before the deadline. Red first: both stream tests ended after 1.01 s / 1.00 s through the session limit. A deliberately over-broad fix (no total for the whole session) fails the write guard. make qa green (lock check, ruff, pyright, bandit, 92 tests).

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