chore: updated AGENTS.md with more testing guidelines

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request updates .agents/AGENTS.md to add guidelines recommending declarative container matchers over imperative loops and custom matchers for complex objects in tests. The review feedback correctly identifies that the provided example uses an undefined matcher HasTimeSeries and suggests a correction to make the example self-contained.

[gemini-code-assist]

The matcher HasTimeSeries is used in the example assertion but is not defined in the code snippet. Since MetricType and HasMetricLabel already inspect arg.metric(), we can remove the undefined HasTimeSeries wrapper to make the example self-contained and correct.

Suggested change

	EXPECT_THAT(recorded_metrics, Contains(HasTimeSeries(AllOf(
	MetricType(HasSubstr("outstanding_rpcs")),
	HasMetricLabel("channel_pool_lb_policy", Eq("RANDOM_TWO_LEAST_USED"))))));
	EXPECT_THAT(recorded_metrics, Contains(AllOf(
	MetricType(HasSubstr("outstanding_rpcs")),
	HasMetricLabel("channel_pool_lb_policy", Eq("RANDOM_TWO_LEAST_USED")))));

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.23%. Comparing base (c49c86f) to head (bd80c18).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16328   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files        2227     2227           
  Lines      209169   209169           
=======================================
+ Hits       192929   192932    +3     
+ Misses      16240    16237    -3     

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[colinmoy]

See gemini bot's suggestion