Add isPty and fold TerminalComplete into ToolResultTerminalContent

[anthonykim1]

Resolves: #322, #323
Part of: microsoft/vscode#324825

• Add optional isPty to ToolResultTerminalContent so a client knows whether a terminal is PTY-backed before subscribing or initializing an emulator.
• Add matching isPty to TerminalState so the channel is self-describing regardless of how a client discovered it; isPty: false means plain-text output and clients do not need to parse VT sequences.
• Fill exitCode, preview, and truncated in on the terminal block in the completed result, so clients that did not subscribe to the terminal still get the command outcome. This records the command's exit, not the terminal's — the terminal may keep running afterwards.
• Remove ToolResultTerminalCompleteContent (breaking, riding the 0.7.0 window): its completion data now lives directly on the terminal block it previously correlated with, so clients no longer pair two content blocks per command.
• Leave cwd off the completion data deliberately — it can be added back additively if a client needs to display it at the top-level terminal part.
• Keep resource and title required. Per review discussion, non-pty shell runs will be exposed as resource-backed terminals (isPty: false) whose output streams over the terminal channel, so clients receive output only when they subscribe.
• Add round-trip fixtures 030-terminal-content-ispty and 031-terminal-content-completed, stage added/removed change fragments, and regenerate the JSON schemas and Go/Rust/Kotlin/Swift/TypeScript clients.

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Inspirations from:

• The completion fields are relocated semantically from the removed ToolResultTerminalCompleteContent — same exitCode/preview/truncated shapes and doc intent, moved onto the terminal block.
• The isPty capability-flag placement on TerminalState follows the existing supportsCommandDetection pattern.
• The streaming path non-pty terminals will use already exists: terminal/data appends into TerminalState.content, so late subscribers reconstruct output from channel state.
• The gap this feeds into is VS Code's tool-name sniffing to decide whether partial results are terminal output: isShellTool — the protocol now labels terminal resources instead.