Remove unused function secrets merge behavior

[sarah (satvu)]

Issue describing the changes in this PR

Resolves #11996

Summary

Removes the unused merged parameter from ISecretManager.GetFunctionSecretsAsync and deletes the associated host/function key merge behavior.

The merged: true path had no production callers and could throw an ArgumentException when function-scoped and host-scoped keys differed only by casing. Tests, mocks, and test implementations have been updated to use the simplified method signature, and the merge-specific test has been removed.

This is not a customer-facing breaking change. ISecretManager is an internal product library that is not consumed directly by customers, and the only caller of the merge behavior was removed previously.

Validation

• Affected unit tests: 145 passed
• Focused integration tests: 3 passed

Pull request checklist

IMPORTANT: Currently, changes must be backported to the in-proc branch to be included in Core Tools and non-Flex deployments.

• Backporting to the in-proc branch is not required
  • Otherwise: Link to backporting PR
• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation
• I have added or updated all required tests

[Copilot]

🟡 Changes recommended

Update the stale coldstart.jittrace reference to the removed overload.

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Pull request overview

Removes the unused merged parameter and host/function secret merge behavior, updating implementations and tests.

Changes:

• Simplifies GetFunctionSecretsAsync.
• Updates mocks, callers, and test implementations.
• Removes merge-specific test coverage.

File summaries

File	Summary
test/WebJobs.Script.Tests/Security/SecretManagerTests.cs	Updates secret manager tests.
test/WebJobs.Script.Tests/Managment/WebFunctionsManagerTests.cs	Updates test callers.
test/WebJobs.Script.Tests/Controllers/Admin/KeysControllerTests.cs	Updates key controller tests.
test/WebJobs.Script.Tests.Shared/TestSecretManager.cs	Updates the test secret manager.
test/WebJobs.Script.Tests.Integration/Management/FunctionsSyncManagerTests.cs	Updates integration tests.
test/WebJobs.Script.Tests.Integration/Controllers/Keys/KeyManagementFixture.cs	Updates key management fixtures.
test/WebJobs.Script.Tests.Integration/Controllers/Keys/GetFunctionKeysScenario.cs	Updates function key scenarios.
src/WebJobs.Script.WebHost/Security/KeyManagement/SecretManager.cs	Removes merge behavior and parameter.
src/WebJobs.Script.WebHost/Security/KeyManagement/ISecretManager.cs	Simplifies the interface contract.

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 1
• Review effort level: Lite

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[Copilot]

🟢 Approval recommended

No unresolved review issues were identified.

Review details

• Files reviewed: 10/10 changed files
• Comments generated: 0 new
• Review effort level: Lite

[sarah (satvu)]

Waiting for #11722 to go in first