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