outlierdetection: convert pick_first health listener test to e2e

[hugehoo]

• Updates outlierdetection: switch as many of the existing tests as possible/feasible to e2e style #8658
• RELEASE NOTES: n/a

Why

• This test directly invokes the outlier detection interval algorithm, coupling it to internal implementation details.

What

• Move the pick_first health listener test to the outlier detection E2E suite.

How

• Configure the policy through the service config, generate failures with real RPCs, and verify the channel remains READY without accessing the balancer internals.

• This is a test-only refactor; no test-first production change is required.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.27%. Comparing base (75f3c0b) to head (c4baead).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9252      +/-   ##
==========================================
+ Coverage   83.23%   83.27%   +0.04%     
==========================================
  Files         421      421              
  Lines       34086    34086              
==========================================
+ Hits        28370    28384      +14     
+ Misses       4281     4268      -13     
+ Partials     1435     1434       -1     

see 30 files with indirect coverage changes

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