@W-21221947: Refactor tests to stub env vars instead of mocking the Config module

[anyoung-tableau]

I initially made these changes as I was working on #216 but pulled them out to isolate them and make the other PR smaller.

[Copilot]

Pull request overview

Refactors unit tests to stub environment variables directly (via Vitest) instead of mocking the Config module, centralizing common env setup in shared test utilities.

Changes:

• Replace vi.mock('../../config.js') patterns across multiple test suites with vi.stubEnv(...) and a shared stubDefaultEnvVars() helper.
• Add stubDefaultEnvVars() to src/testShared.ts and use it from src/testSetup.ts and updated tests.
• Update src/config.test.ts to use vi.stubEnv/vi.unstubAllEnvs rather than manual process.env mutation.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
src/tools/workbooks/getWorkbook.test.ts	Switch config mocking to env stubbing via stubDefaultEnvVars() and per-test vi.stubEnv.
src/tools/views/getViewImage.test.ts	Switch config mocking to env stubbing via stubDefaultEnvVars() and per-test vi.stubEnv.
src/tools/views/getViewData.test.ts	Switch config mocking to env stubbing via stubDefaultEnvVars() and per-test vi.stubEnv.
src/tools/queryDatasource/queryDatasource.test.ts	Replace config mocking with env stubbing for datasource credentials and bounded-context filtering.
src/tools/queryDatasource/datasourceCredentials.test.ts	Replace config mocking with DATASOURCE_CREDENTIALS env stubbing.
src/tools/pulse/generateMetricValueInsightBundle/generatePulseMetricValueInsightBundleTool.test.ts	Replace config mocking with env stubbing for bounded-context datasource filtering.
src/tools/pulse/generateInsightBrief/generatePulseInsightBriefTool.test.ts	Replace config mocking with env stubbing for bounded-context datasource filtering.
src/tools/getDatasourceMetadata/getDatasourceMetadata.test.ts	Replace config mocking with env stubbing for feature flags and bounded-context datasource filtering.
src/testShared.ts	Introduce stubDefaultEnvVars() helper for shared env defaults.
src/testSetup.ts	Use stubDefaultEnvVars() instead of duplicating env stubs inline.
src/telemetry/init.test.ts	Replace config mocking with TELEMETRY_PROVIDER / TELEMETRY_PROVIDER_CONFIG env stubbing.
src/config.test.ts	Refactor env setup/teardown to use vi.unstubAllEnvs + vi.stubEnv rather than rewriting process.env.

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