fix(terminal-chat): dedupe terminal tool-session registrations to prevent listener leak (#309906)

[maruthang]

What
Dedupe terminal tool-session registrations in TerminalChatService.registerTerminalInstanceWithToolSession so repeated registrations for the same terminal instance don't stack up event listeners.

Why
RunInTerminalTool.invoke() generates a fresh terminalToolSessionId per tool call and re-registers the same terminal instance each time. The existing implementation subscribed to chatService.onDidDisposeSession and instance.onDisposed on every call without disposing the prior listeners, so listeners accumulated on shared emitters until the leak detector tripped. This is the root cause behind the telemetry bucket [Unhandled Error] potential listener LEAK detected, dominated reported in #309906 (error-telemetry, recent-regression).

How
Added a dedupe guard at the top of registerTerminalInstanceWithToolSession:

• If the instance is already registered with the same terminalToolSessionId, early-return (no-op).
• Otherwise, deleteAndDispose the stale listener and drop the stale mapping before registering the new one.

This mirrors the pattern already used by the sibling registerTerminalInstanceWithChatSession in the same file, keeping the two registration paths consistent.

Test plan
Added src/vs/workbench/contrib/terminalContrib/chat/test/browser/terminalChatService.test.ts with two regression tests that inspect Emitter._size (the same counter the leak detector uses):

• Asserts the listener count on onDidDisposeSession stays ≤ 1 after 50 re-registrations with fresh tool-session ids.
• Asserts re-registering with the same id is a no-op and doesn't add a listener.

npm run compile-check-ts-native passes cleanly for this change.

Fixes #309906

[meganrogge]

Thanks!