Retire completed non-PTY shell output resources

[anthonykim1]

Resolves: #327178
Follow-up to: #327067

• Add idempotent retirement for completed non-PTY shell output resources, removing both the host resource and its tracked stream.
• Publish the final ChatToolCallComplete result before retiring the live output resource so completed rendering has an authoritative static result.
• Retire only when both an exit code and preview are available; an empty preview still counts as authoritative.
• Keep background and no-preview output streams session-owned until normal session disposal.
• Prevent completed static non-PTY results from attaching to an already-retired URI, while preserving normal turn-owned cleanup for attachments created during streaming.
• Add coverage for distinct shell-call URIs, empty previews, background output, session cleanup, completed history, and PTY compatibility.

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Inspirations from:

• Output-only terminal retirement uses the existing non-process cleanup path in AgentHostTerminalManager.disposeTerminal.
• Completed terminal rendering already prefers structured result.preview through getTerminalOutput.
• Static terminal identity and output restoration follow buildTerminalToolSpecificData.

[Copilot]

Pull request overview

Retires completed non-PTY shell output resources once authoritative static results are available, preventing per-session resource growth.

Changes:

• Adds idempotent host-side retirement after publishing completion.
• Avoids attaching completed static results to retired resources.
• Expands lifecycle and compatibility coverage.

Show a summary per file

File	Description
agentHostChatContribution.test.ts	Tests static rendering and attachment cleanup.
agentHostSessionHandler.ts	Skips live attachment for authoritative static results.
copilotAgentSession.test.ts	Covers retirement and retained-stream scenarios.
copilotNonPtyShellTerminals.ts	Implements retirement and tracking cleanup.
copilotAgentSession.ts	Retires resources after publishing completion.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Medium