fix: improve terminal output capture for first command in new terminals

[roomote]

Description

This PR attempts to address Issue #9019 where terminal command output is not reliably captured when it's the first command in a freshly created terminal session. The issue is particularly noticeable with chained commands like ENV=hello && echo $ENV.

Problem

There's a race condition where the shell integration stream might not be fully ready when the first command is executed in a newly created terminal, causing the output to be lost even though the command executes successfully.

Solution

This fix implements a two-pronged approach:

1. Terminal initialization delay: Added a 200ms delay for the first command in newly created terminals to ensure shell integration is fully ready
2. Improved stream handling: Enhanced the stream data processing to better handle cases where shell integration markers are delayed or missing

Changes

• Added tracking for newly created terminals (isNewlyCreated flag)
• Implemented a one-time 200ms delay before executing the first command in new terminals
• Fixed logic error in stream chunk tracking (replaced streamDataReceived with chunksReceived counter)
• Added fallback logic to treat accumulated data as output if markers aren't found after 500 characters
• Added comprehensive documentation explaining timing values and rationale

Testing

• All existing terminal tests pass
• The fix addresses the reproduction case: ENV=hello && echo $ENV now reliably captures output
• No regression in existing terminal functionality

Notes

• The 200ms delay and 500-character threshold are empirically chosen values that balance reliability with performance
• These values may need adjustment for different systems or shells
• Future work could make these values configurable

Fixes #9019

Feedback and guidance are welcome!

---

Important

Fixes terminal output capture race condition by adding delay for first command and improving stream handling in new terminals.

• Behavior:
  • Adds 200ms delay for first command in new terminals in Terminal.ts to ensure shell integration readiness.
  • Improves stream data handling in TerminalProcess.ts to manage delayed or missing shell integration markers.
• Implementation:
  • Introduces isNewlyCreated and firstCommandExecuted flags in Terminal class.
  • Replaces streamDataReceived with chunksReceived counter in TerminalProcess.
  • Adds fallback logic in TerminalProcess to treat data as output if markers aren't found after 500 characters.
• Testing:
  • All existing tests pass; fix confirmed for ENV=hello && echo $ENV case.
  • No regression in terminal functionality.

^{This description was created by for 087ddf2. You can customize this summary. It will automatically update as commits are pushed.}

[roomote]

Review complete. The implementation correctly addresses the terminal output capture race condition with a 200ms delay for first commands in new terminals and improved fallback logic. Patch 2 fixes the logic error from patch 1 by replacing the always-false boolean check with a chunk counter. No issues found.

_{Mention @roomote in a comment to trigger your PR Fixer agent and make changes to this pull request.}