Optimize slow query logger to reduce CPU usage

[apurvabhaleMS]

Description

Optimizes LogQueryStoreByTextAsync in SqlServerSearchService to reduce CPU
impact during transient slowdowns.

Changes:

• Hash-based Query Store lookup: Leverages the /* HASH ... */ comment already
  embedded by SqlQueryGenerator to match queries via hash instead of expensive
  REPLACE+LIKE on query_sql_text. Falls back to text matching for queries without hashes.
• Stored procedure path: Uses OBJECT_ID lookup instead of text scanning.
• Timeout cap: 2s cancellation token + 5s command timeout on diagnostic queries
  so a slow lookup never blocks for long.
• lookup= diagnostic tag: Logs which lookup method was used
  (Hash, TextNoHash, TextFallback, StoredProc) for observability.
• Unit tests for hash extraction and the diagnostic timeout constant.

Related issues

Addresses AB191430

Testing

Describe how this change was tested.

FHIR Team Checklist

• Update the title of the PR to be succinct and less than 65 characters
• Add a milestone to the PR for the sprint that it is merged (i.e. add S47)
• Tag the PR with the type of update: Bug, Build, Dependencies, Enhancement, New-Feature or Documentation
• Tag the PR with Open source, Azure API for FHIR (CosmosDB or common code) or Azure Healthcare APIs (SQL or common code) to specify where this change is intended to be released.
• Tag the PR with Schema Version backward compatible or Schema Version backward incompatible or Schema Version unchanged if this adds or updates Sql script which is/is not backward compatible with the code.
• When changing or adding behavior, if your code modifies the system design or changes design assumptions, please create and include an ADR.
• CI is green before merge
• Review squash-merge requirements

Semver Change (docs)

Patch|Skip|Feature|Breaking (reason)

[Copilot]

Pull request overview

This PR optimizes the slow-query Query Store diagnostic logger in SqlServerSearchService to reduce CPU overhead during transient database slowdowns by avoiding expensive text normalization scans and adding throttling/backoff behavior.

Changes:

• Adds a circuit breaker/backoff around Query Store diagnostic lookups to prevent repeated expensive failures during incidents.
• Uses faster Query Store lookup strategies: stored-proc lookup via OBJECT_ID, and ad-hoc lookup via embedded /* HASH ... */ comment with fallback to prior text matching.
• Adds unit tests covering parameter-hash extraction and validating throttling/timeout constants.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Microsoft.Health.Fhir.SqlServer/Features/Search/SqlServerSearchService.cs	Adds hash-based and stored-proc Query Store lookup paths, circuit breaker/backoff, and lookup-method tagging in logged stats.
src/Microsoft.Health.Fhir.SqlServer.UnitTests/Features/Search/SqlServerSearchServiceQueryStoreTests.cs	Adds tests for ExtractParameterHash and sanity checks for throttling/timeout constants.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.13%. Comparing base (3e191c5) to head (3b19103).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5601      +/-   ##
==========================================
- Coverage   77.29%   77.13%   -0.16%     
==========================================
  Files         997      997              
  Lines       36564    36693     +129     
  Branches     5549     5559      +10     
==========================================
+ Hits        28261    28304      +43     
- Misses       6928     7016      +88     
+ Partials     1375     1373       -2     

see 7 files with indirect coverage changes

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[mikaelweave]

a couple comments