agent: Improve Gemini support in the edit_file tool (#31116)

This change improves `eval_extract_handle_command_output` results for
all models:

Model                       | Pass rate before | Pass rate after
----------------------------|------------------|----------------
claude-3.7-sonnet           |  0.96            | 0.98
gemini-2.5-pro              |  0.35            | 0.86
gpt-4.1                     |  0.81            | 1.00

Part of this improvement comes from more robust evaluation, which now
accepts multiple possible outcomes. Another part is from the prompt
adaptation: addressing common Gemini failure modes, adding a few-shot
example, and, in the final commit, auto-rewriting instructions for
clarity and conciseness.

This change still needs validation from larger end-to-end evals.


Release Notes:

- N/A

Cargo.lock

@@ -4402,6 +4402,15 @@ version = "0.1.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "56254986775e3233ffa9c4d7d3faaf6d36a2c09d30b20687e9f88bc8bafc16c8"
 
+[[package]]
+name = "diffy"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b545b8c50194bdd008283985ab0b31dba153cfd5b3066a92770634fbc0d7d291"
+dependencies = [
+ "nu-ansi-term 0.50.1",
+]
+
 [[package]]
 name = "digest"
 version = "0.10.7"
@@ -8677,6 +8686,7 @@ dependencies = [
  "clock",
  "collections",
  "ctor",
+ "diffy",
  "ec4rs",
  "env_logger 0.11.8",
  "fs",
@@ -10191,6 +10201,15 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "nu-ansi-term"
+version = "0.50.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d4a28e057d01f97e61255210fcff094d74ed0466038633e95017f5beb68e4399"
+dependencies = [
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "num"
 version = "0.4.3"
@@ -16389,7 +16408,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e8189decb5ac0fa7bc8b96b7cb9b2701d60d48805aca84a238004d665fcc4008"
 dependencies = [
  "matchers",
- "nu-ansi-term",
+ "nu-ansi-term 0.46.0",
  "once_cell",
  "regex",
  "serde",