zulip: Reset session on ConnectionError to fix stale connections.

[HeetRanpura]

Overview

When a network middlebox silently drops an idle TCP connection,
requests.Session keeps that dead socket in its connection pool. The next
API call tries to reuse it and raises a ConnectionError. Previously the
retry loop in do_api_query caught this error but never closed the broken
session, so every retry attempt reused the same dead connection and failed.

Root Cause

ensure_session() was called once before the while True loop, so after a
ConnectionError the session was never rebuilt on retry. All 10 retries
attempted to send requests over the same stale TCP connection.

Fix

Two minimal changes to do_api_query:

1. Move ensure_session() inside the while True loop so it runs on every
2. iteration.
3. 2. On ConnectionError, close and null out self.session before retrying.
4. The next iteration calls ensure_session(), sees None, and creates a
5. fresh connection.

if self.session is not None:
    self.session.close()
self.session = None

The existing error_retry machinery is reused completely unchanged. No new
parameters, timeouts, or retry counters were added.

Why This Approach

PR #854 added a parallel retry system with new timeouts and keep-alive
headers - too broad. PR #913 was on the right track (session reset) but
had linting failures and stalled. This PR takes the same minimal approach
with full lint compliance and proper test coverage.

Test Results

============================= test session starts ==============================
platform darwin -- Python 3.9.6, pytest-8.4.2

zulip/tests/test_do_api_query.py::TestDoApiQuery::test_session_not_reset_when_never_connected PASSED [ 50%]
zulip/tests/test_do_api_query.py::TestDoApiQuery::test_session_reset_on_connection_error PASSED [100%]

========================= 2 passed in 2.14s ============================

Full project suite (437 tests, 0 failures):

========================= 437 passed, 5 warnings in 4.57s ========================

Linting (ruff + mypy):

mypy | Success: no issues found in 72 source files
ruff | All checks passed.

Gitlint:

gitlint --msg-filename /dev/stdin   # exit 0, no violations

Tests Added

zulip/tests/test_do_api_query.py:

• test_session_reset_on_connection_error - First session raises
• ConnectionError; verifies the stale session is close()d, a fresh
• session is created, and the request succeeds on retry.
• • test_session_not_reset_when_never_connected - ConnectionError before
• first successful connection raises UnrecoverableNetworkError without
• touching the session.
  Fixes Improve error handling when a request fails due to the persistent connection being expired  #761.