[CoR] Resolve deployment plans from the active session

[nturinski]

Problem

The deployment-plan view currently chooses the most recently modified prepare-plan.json. An older session can therefore appear instead of the active deployment session if its artifact has a newer timestamp. This can briefly show the original or otherwise incorrect project plan before the expected plan opens.

Proposed work

• Resolve .copilot-azure/sessions/active-session.json first.
• Prefer .copilot-azure/sessions/{activeSessionId}/prepare-plan.json and verify that any root-level plan belongs to the active session.
• Use newest-modification-time selection only as a recovery fallback when the active-session pointer is absent or stale.
• Apply the same active-session filtering to plan watchers so a non-active session cannot steal focus.
• Extract a shared active-session artifact resolver rather than duplicating logic.

Existing patterns to reuse:

• src/webviews/copilotOnRails/extension/openDeployResultView.ts
• src/webviews/copilotOnRails/extension/deployProgressWatcher.ts

Primary implementation surface:

• src/webviews/copilotOnRails/extension/openDeploymentPlanView.ts
• Potential shared helper in src/tree/project/projectPlanFiles.ts

Acceptance criteria

• Opening deployment planning selects the active session's plan whenever a valid active-session pointer exists.
• Updating an older session's plan cannot replace or focus over the active plan.
• A missing or stale pointer still has a deterministic recovery path.
• Existing approval state keyed by artifact path/content remains intact.
• Tests cover active, stale, missing, and competing-session artifacts.