Return final answer text when snippet hydration errors

[guomaggie]

Sometimes the model will return the hydrated snippet in a format that doesn't get parsed correctly.
If any parsing error occurs, the model should fail gracefully by appending the unparsed lines as-is so the main agent is still getting some information.

Testing:

1. Raw subagent response no longer includes error after file + line, and we return the original unparsed line

2. Misformatted response (with unrecognized file outside final answer tags)

3. Misformatted response (with unrecognized file inside final answer tags)

[Copilot]

Pull request overview

Improves resilience of the Copilot search subagent snippet “hydration” step so that when the model output can’t be parsed or a referenced file can’t be opened, the tool returns the original text instead of appending a noisy error suffix. This helps the main agent still receive useful information even when the subagent response format drifts.

Changes:

• Update parseFinalAnswerAndHydrate to keep the original response line verbatim when hydration fails.
• Add unit tests covering (1) non-matching line formats and (2) matching lines that fail to open/hydrate, ensuring no error suffix is appended.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
extensions/copilot/src/extension/tools/node/searchSubagentTool.ts	On hydration failure, preserves the model’s original line instead of adding an “unable to read file” suffix.
extensions/copilot/src/extension/tools/node/test/searchSubagentTool.spec.ts	Adds coverage to ensure misformatted or unopenable snippet references are returned verbatim.