fix(connect): harden websocket reconnect and reply delivery

[darwin67]

Summary

• guard Connect websocket send failures so stale sockets can trigger reconnect without clearing a replacement connection
• abandon executor requests when WORKER_REQUEST_ACK cannot be sent, letting the gateway retry instead of creating a synthetic SDK execution failure
• keep buffered replies after failed /v0/connect/flush attempts so the fallback delivery path can retry

Review structure

• d61a238 guards stale websocket send failures and adds safe_send coverage
• 3108435 hardens request ACK and flush handling with execution-handler coverage

Verification

• uv run --extra dev pytest -q pkg/inngest/inngest/connect/_internal
• uv run ruff check on touched files
• uv run ruff format --check on touched files
• cd pkg/inngest && uv run mypy --config-file=../../mypy.ini .

[linear-code]

SYS-856