fix: defer model disposal until after input state flush when switching chat threads (#309115)

[maruthang]

Summary

Fixes #309115

Bug: Chat input prompt text was not saved when switching between threads, causing the user's in-progress prompt to be lost.

Root Cause: In ChatViewPane.showModel(), the old model reference was disposed immediately via this.modelRef.value = undefined, which triggered model serialization before the widget's setModel call could flush the current input state to the outgoing model.

Fix: Use clearAndLeak() to extract the old reference without disposing it, then manually dispose it after setModel has been called (which triggers the input state flush). Early-return and error paths also properly dispose the old reference to prevent leaks.

Changes

• src/vs/workbench/contrib/chat/browser/widgetHosts/viewPane/chatViewPane.ts: Replace immediate this.modelRef.value = undefined with clearAndLeak() pattern that defers disposal until after setModel flushes input state. Added proper disposal in all early-return and error paths.
• src/vs/workbench/contrib/chat/test/common/model/chatModelStore.test.ts: Added 2 regression tests validating the disposal ordering behavior for issue Agents: Regression in chat input prompt being saved #309115.

Testing

• Added regression test verifying model stays alive when reference is leaked and disposed after state flush
• Added regression test demonstrating the problematic early-release pattern that caused the bug
• All existing tests pass