fix: resolve connection leaks, pending request tracking, and infinite combo loops

[pandamoon21]

Problem

This PR fixes three request-handling stability issues:

• Client disconnects were not propagated to upstream fetch execution, which could leave sockets open and eventually make the HTTP port unresponsive.
• Successful streaming responses did not immediately decrement the pending request counter, so completed requests could stay counted as pending until the fallback timeout.
• Combo models could reference themselves directly or indirectly, causing infinite recursion, 100% CPU usage, and a hung server.

Fix

• Pass the incoming request's AbortSignal through the chat handling flow and wire it into the stream controller disconnect/abort path.
• Return client-aborted 499 responses immediately so they do not mark healthy accounts unavailable or trigger account fallback.
• Add an onComplete callback so successful streams decrement the pending request counter immediately.
• Track visited combo models during expansion and return 400 Bad Request when a circular combo reference is detected.

Regression coverage

• Client disconnect propagation and abort listener cleanup
• Client aborts do not mark accounts unavailable or trigger fallback
• Pending request decrement on successful stream completion
• Direct and indirect circular combo detection
• Safe 400 Bad Request response for circular combo references

Verification

• npx eslint "src/sse/handlers/chat.js" "tests/unit/chat-client-abort-fallback.test.js" — passed
• cd tests && npx vitest run unit/chat-client-abort-fallback.test.js unit/combo-routing.test.js unit/combo-fusion.test.js unit/gemini-native-endpoint.test.js unit/headroom-chat-core.test.js — 28/28 passed
• git diff --check — passed