fix(sdk): raise asyncio StreamReader buffer in Python AsyncHostTransport

[michaelreavant]

Summary

superdoc_get_content on larger documents fails in the Python async SDK with a misleading HOST_DISCONNECTED error. Root cause is a missing limit= on the subprocess spawn — the stdout StreamReader inherits asyncio's 64 KiB default buffer, which any non-trivial response trips. This PR raises the buffer ceiling and exposes it as a configurable option.

Problem

The Python async transport spawned the host CLI without passing a limit= to asyncio.create_subprocess_exec, so its stdout StreamReader inherited asyncio's default 64 KiB buffer. Every host response is written as a single newline-delimited JSON line, so any cli.invoke whose serialized result exceeds 64 KiB (e.g. superdoc_get_content on larger documents) caused readline() to raise ValueError: Separator is not found, and chunk exceed the limit inside _reader_loop. The exception was caught by the generic reader-loop handler and pending requests were rejected with the misleading HOST_DISCONNECTED error — even though the host process was still alive and healthy.

Fix

Pass limit= to create_subprocess_exec and expose it as a new stdout_buffer_limit_bytes constructor option on AsyncHostTransport, threaded through SuperDocAsyncRuntime and AsyncSuperDocClient. The default of 64 MiB safely covers the host's own 32 MiB DEFAULT_MAX_STDIN_BYTES input cap with room for ~2x JSON expansion.

Scope

SyncHostTransport is unaffected — it uses raw blocking subprocess.Popen which has no asyncio buffer limit. No changes to the Node SDK or the host server.

Tests

Adds a TestAsyncLargeResponse regression suite that:

1. Round-trips a 200 KB response through the default-configured transport.
2. Pins that an explicitly tightened stdout_buffer_limit_bytes still reproduces the original failure mode, guaranteeing the option is wired through to create_subprocess_exec.

Bug reproduction was verified by stashing the fix and running the new test against the unmodified transport.py — it raised the exact SuperDocError: Host process disconnected. seen in production. With the fix in place, the full Python SDK test suite passes (90 tests, including 26 transport tests).

Test plan

• pytest packages/sdk/langs/python/tests/ — 90 passed
• Verified regression test reproduces the original HOST_DISCONNECTED failure when the fix is reverted
• pnpm run generate:all clean (no codegen drift in this branch)