Consolidate gremlin-python read timeout into a single ReadTimeoutError

[GumpacG]

gremlin-python armed two read-timeout mechanisms at the same value - the driver's
async_timeout wrapper (raising asyncio.TimeoutError) and aiohttp's sock_read
(raising SocketTimeoutError). Whichever fired first was a race, so a read timeout
surfaced nondeterministically as one type or the other, which also made
test_client_side_timeout_set_for_aiohttp flaky (it asserted on the exception message).

Change: remove the redundant async_timeout read wrapper and keep aiohttp sock_read
as the single read-timeout source (it also covers the initial-response wait and is
streaming-safe). Normalize its SocketTimeoutError/ServerTimeoutError into a new
driver-owned ReadTimeoutError so a read timeout always surfaces as one deterministic,
transport-agnostic type. ReadTimeoutError subclasses the builtin TimeoutError, so it
stays catchable via except TimeoutError while leaking neither asyncio nor aiohttp types
(addresses review feedback). It is documented as temporary: once the driver is fully
async it should revert to asyncio.TimeoutError (TINKERPOP-2774).

Assisted-by: Kiro: Claude Opus 4.8

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.41%. Comparing base (a28cd1f) to head (73d8df2).
⚠️ Report is 204 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3507      +/-   ##
============================================
+ Coverage     76.35%   76.41%   +0.05%     
- Complexity    13424    13924     +500     
============================================
  Files          1012     1027      +15     
  Lines         60341    62630    +2289     
  Branches       7075     7355     +280     
============================================
+ Hits          46076    47856    +1780     
- Misses        11548    11793     +245     
- Partials       2717     2981     +264     

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[kenhuuu]

VOTE +1

[kenhuuu]

merging as lazy consensus reached