fix: prevent intermittent connection resets in internal service client (#4971)

[krxshes]

What does this PR address?

Fixes intermittent httpx.RemoteProtocolError/aiohttp connection resets when calling a dependent service via bentoml.depends() (e.g. ServiceA → ServiceB), surfaced to callers as unhandled 500s.

Two related issues in SessionManager/_call (_bentoml_impl/client/proxy2.py):

1. The tcp-scheme branch of _make_client() left connector=None, so aiohttp fell back to its default keepalive_timeout (15s), which exceeds uvicorn's default timeout_keep_alive (5s). A pooled connection could be closed server-side while the client still considered it reusable, causing writes to fail mid-request.
2. _call() had no retry on connection-level failure. Added a retry-once path that rebuilds the request payload (rather than reusing the original, since it can be a one-shot async iterator/file stream) before retrying.

Fix:

• Explicitly set keepalive_timeout=4.0 on the TCPConnector used for tcp-scheme sessions, keeping it safely under the server's 5s default.
• Wrap the outbound request in a targeted retry that catches aiohttp.ClientConnectionError/aiohttp.ServerDisconnectedError and retries once with a freshly rebuilt payload.

Testing:

• Added a regression test in tests/unit/_internal/client/test_session_manager.py asserting the connector's keepalive_timeout.
• Verified with a local load-test reproduction (two dependent @bentoml.service instances, bursts of 200 requests timed against the keep-alive boundary): before this fix, this reliably produced a cluster of 500s; after, zero failures across repeated runs.

Fixes #4971

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[krxshes]

Hi @bojiang, just following up to see if you have time for a quick review on this fix. Thanks!