[Cherry-Pick] Remove IStorageSourceFilter (#323480)

[benvillalobos]

Related: #324669

This is a cherry-pick of PR #323480 merge commit.

Context

#323914 started from a base that existed before #323480. Merge conflict resolution accidentally ended with the version prior to #323480 being accepted.

A single merge conflict was resolved by accepting main's code, no manual change needed.

[Copilot]

Pull request overview

This PR cherry-picks #323480, which removes the IStorageSourceFilter abstraction and its associated helpers from the AI Customization / chat harness infrastructure. Previously, each harness descriptor exposed a getStorageSourceFilter(type) method and items were post-filtered by source; since all harnesses returned the full source set, the filter was effectively a no-op, so the mechanism is deleted to simplify the code. The cherry-pick was needed because a merge-conflict resolution in #323914 accidentally reverted to the pre-#323480 state.

Changes:

• Remove the IStorageSourceFilter interface, the applySourceFilter/applyStorageSourceFilter helpers, and the getStorageSourceFilter member from IHarnessDescriptor.
• Drop the now-unused getActiveDescriptor constructor parameter from PromptsServiceCustomizationItemProvider (and its applyLocalFilters method), updating all 3 call sites.
• Remove the filter definitions from all harness descriptor factories/tests and clean up the corresponding imports.

Show a summary per file

File	Description
common/aiCustomizationWorkspaceService.ts	Removes IStorageSourceFilter interface and the applySourceFilter/applyStorageSourceFilter helpers; drops unused PromptsStorage import.
common/customizationHarnessService.ts	Removes getStorageSourceFilter from IHarnessDescriptor, the EMPTY_FILTER constant, and descriptor filter wiring.
browser/aiCustomization/promptsServiceCustomizationItemProvider.ts	Drops getActiveDescriptor ctor param and applyLocalFilters; stops applying the source filter.
browser/aiCustomization/aiCustomizationItemsModel.ts	Updates provider instantiation to drop the descriptor arg.
browser/aiCustomization/aiCustomizationManagementEditor.ts	Updates provider instantiation to drop the descriptor arg.
browser/aiCustomization/customizationCreatorService.ts	Updates provider instantiation to drop the descriptor arg.
browser/agentSessions/agentHost/agentHostChatContribution.ts	Removes getStorageSourceFilter from descriptor and unused import.
api/browser/mainThreadChatAgents2.ts	Removes getStorageSourceFilter from extension-provided harness descriptor.
sessions/.../remoteAgentHost/browser/remoteAgentHostCustomizationHarness.ts	Removes filter construction and getStorageSourceFilter from the remote harness descriptor.
sessions/.../aiCustomizationShortcutsWidget.fixture.ts	Removes getStorageSourceFilter from the mock descriptor.
test/.../customizationHarnessService.test.ts	Removes getStorageSourceFilter usages and the filter assertion.
test/.../aiCustomizationListWidget.test.ts	Removes getStorageSourceFilter mock and unused imports.
test/.../aiCustomizationItemsModel.test.ts	Removes getStorageSourceFilter mocks and unused IStorageSourceFilter import.
sessions/.../remoteAgentHostCustomizationHarness.test.ts	Removes getStorageSourceFilter mock and unused import.

Review details

• Files reviewed: 14/14 changed files
• Comments generated: 0
• Review effort level: Medium