fix: improve vendor configuration handling in LanguageModelsService

[DonJayamanne]

Fixes #307898

[Copilot]

Pull request overview

This PR adjusts how LanguageModelsService handles vendor/group configuration resolution, aiming to avoid unnecessary model re-resolution for groups that only contribute per-model settings and to ensure per-model config can be persisted even when a vendor is hidden by its when clause.

Changes:

• Changes group handling during model resolution so vendors without a vendor-level configuration schema don’t automatically trigger separate per-group model resolution.
• Updates per-model config persistence to look up vendors from the full _vendors map (not getVendors()), so config can be stored even if a vendor is currently hidden by when.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/common/languageModels.ts	Tweaks model resolution flow for schema-less vendors and changes vendor lookup used when persisting per-model settings.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/common/languageModels.ts:895

• The new if (!vendor.configuration) shortcut skips per-group provideLanguageModelChatInfo({ group: ... }) calls for all vendors that don’t contribute a vendor-level configuration schema. That’s a behavior change vs the previous vendor.isDefault && !vendor.configuration guard, and it breaks providers that only return models when a group is specified (e.g. the existing per-model configuration tests use a provider that returns [] for the ungrouped call and models only when options.group is set).

Consider narrowing this optimization so group-based resolution still happens when needed (for example, only skip group resolution when the initial ungrouped call already returned models, or keep this special-cased to the default vendor). Otherwise configured groups for schema-less vendors will never resolve any models.

				// For vendors without a configuration schema, groups only carry per-model
				// settings and should not trigger a separate model resolution call.
				// Instead, apply the per-model config to the already-resolved models.
				if (!vendor.configuration) {
					if (group.settings) {
						for (const model of allModels) {
							const modelConfig = group.settings[model.metadata.id];
							if (modelConfig) {
								// Store raw config (without resolving secrets) to avoid leaking secrets on persist
								perModelConfigurations.set(model.identifier, { ...modelConfig });
							}
						}
					}
					languageModelsGroups.push({ group, modelIdentifiers: [] });
					continue;
				}

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