sessions: decouple sessions layer from agent sessions dependencies (#306132)

* refactor: remove unused import and update session status check in SessionsManagementService

* refactor: remove public getGitHubContext method and change its visibility to private in SessionsManagementService

* refactor: remove getGitHubContextForSession method and its fallback logic in SessionsManagementService

* refactor: replace pullRequest with gitHubInfo in session data structures and related logic

* refactor: rename ISessionGitHubInfo to IGitHubInfo and update references across session data structures

* refactor: update GitHub info structure to encapsulate pull request details

* refactor: update session management to utilize gitHubInfo for pull request details

* refactor: simplify file URI resolution in CodeReviewService and remove unused method from SessionsManagementService

* refactor: remove unused getSessionRepositoryUri method and related logic from SessionsManagementService

* refactor: update session type constants to use string literals in sessionTypes and sessionsManagementService

* refactor: update pull request handling to utilize gitHubInfo in ChangesViewPane

* refactor: replace AgentSessionProviders with session type constants across sessions layer

Replace direct imports of AgentSessionProviders enum from the workbench
agent sessions layer with CopilotCLISessionType/CopilotCloudSessionType
constants from the sessions layer's own sessionTypes module.

Simplify workspaceFolderManagement to use session workspace data instead
of provider-specific branching. Remove fileTreeView component.

Co-authored-by: Copilot <[email protected]>

* refactor: remove agent sessions dependencies from changes and mode picker

- changesTitleBarWidget: use ISessionsManagementService instead of
  IAgentSessionsService for session lookup and change events
- changesView: use sessionManagementService.onDidChangeSessions signal
- modePicker: replace AgentSessionProviders with CopilotCLISessionType

Co-authored-by: Copilot <[email protected]>

* fix: add missing gitHubInfo to mock session in codeReviewService tests

Co-authored-by: Copilot <[email protected]>

* fix compilation

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Co-authored-by: Copilot <[email protected]>