Fix custom configuration provider regression

[Sean McManus (sean-mcmanus)]

Summary

Restore canProvideConfiguration calls for each requested URI before invoking the batched provideConfigurations request. This preserves provider initialization and filtering behavior while retaining the multi-file batching introduced in #14582.

Add integration coverage for a provider that prepares its file configuration during canProvideConfiguration, and run the containing SimpleCppProject scenario in CI on Windows, Linux, and macOS.

Fixes #14724.

This PR was investigated and created by Copilot with GPT-5.6 Sol (in VS Code). Any message starting with ✨Copilot: was sent by Copilot.

[Copilot]

🟢 Approval recommended

The implementation restores prior provider behavior and includes targeted regression coverage.

Pull request overview

Restores per-URI provider initialization and filtering while retaining batched configuration requests.

Changes:

• Calls canProvideConfiguration before batching supported URIs.
• Adds integration coverage for provider initialization ordering.

File summaries

File	Description
Extension/src/LanguageServer/client.ts	Filters and initializes URIs before requesting configurations.
Extension/test/scenarios/SimpleCppProject/tests/languageServer.integration.test.ts	Verifies initialization precedes configuration provisioning.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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[Sean McManus (sean-mcmanus)]

✨Copilot (agent50): Reviewed source e7240568 merged over main at 2f25880c (provider merge 79b663e2; parents confirmed).

Root cause verified rather than assumed: #14582 removed the canProvideConfiguration call, and the provider named in #14724 populates its foundFiles cache as a side effect of that call, so its provideConfigurations returned an empty array and IntelliSense fell back to the host compiler with no provider include paths. Restoring the call before the batched request fixes that, and the restored semantics match the pre-#14582 code exactly — including the subtle case where a thrown canProvideConfiguration still leaves the URI treated as supported. Filtering to supportedUris is safe for partial batches, since sendCustomConfigurations is keyed per URI.

Two non-blocking comments below. I could not run the SimpleCppProject scenario locally (extension dependencies and native test binaries are not deployed in this worktree), so the test change is reviewed statically; current-head CI covers compile, lint, and unit tests only.

[Copilot]

🟢 Approval recommended

The implementation restores the previous provider contract with focused cross-platform regression coverage.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Bob Brown (bobbrow)]

One optional comment

[Copilot]

🟢 Approval recommended

The implementation restores the expected provider lifecycle and includes appropriate cross-platform regression coverage.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Sean McManus (sean-mcmanus)]

✨Copilot (agent50): Re-reviewed at 3e8d1092 (merge 71576369 over main 2857a071; parents confirmed). The delta since 75a3078c is whitespace-only — git diff -w between the two revisions is empty — so there is no behavioral change to re-validate.

Current-head CI is fully green on all three platforms, and I confirmed the individual steps rather than the job conclusions: Compile Sources, Run Linter, Run unit tests, and Run SimpleCppProject tests all succeeded on windows-2025, ubuntu-24.04, and macos-15. That is stronger evidence than my earlier single-platform local runs, and it means the regression guard added here now demonstrably runs and passes everywhere.

One non-blocking nit below about the scope of the new hunk. The fix itself remains correct and, from my side, ready to merge.