[FEATURE]: Custom tools: Support incremental output streaming during execution

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Describe the enhancement you want to request

Problem

Custom tools can only return a complete result from execute() — there's no mechanism to stream incremental output to the TUI while the tool is running. Built-in tools like bash stream stdout/stderr progressively, but custom tools show nothing until they finish.

Current behavior

1. tool-call event → tool enters running state
2. (silence)
3. tool-result event → full output appears at once

Expected behavior

Custom tools should be able to emit incremental output during execution, rendered progressively in the TUI.

Use case

Any custom tool that dispatches long-running commands (test suites, builds, deployments, remote execution) produces output over minutes. The user needs to see progress to know the tool is working and to catch issues early.

Possible approaches

1. Streaming HTTP response — the tool reads a chunked/SSE response and the framework renders chunks as they arrive
2. Plugin API extension — add a stream callback to ToolContext that the tool can call incrementally (similar to how ctx.metadata() works for metadata)
3. Polling — the tool periodically updates output via a new ctx.output() method

Context

The bash tool achieves streaming through special processor handling that captures child process stdout/stderr incrementally. Custom tools have no equivalent path — execute() returns Promise<ToolResult>, a single value. The ctx.metadata() method (added in #7590) streams metadata but not the actual output content.