fix(cli): stream agent output as plain text instead of Rich markup

[michaelxu2288]

Problem

In agentex agents run, stream_process_output is the only reader of an agent's stdout pipe. It printed each line with

console.print(f"[dim]{prefix}:[/dim] {decoded_line}")

so the agent's own output was parsed as Rich markup. That has three effects:

1. Markup-like output stops the reader. A line with an unmatched closing tag, such as [INST] hi [/INST] (common in prompt dumps) or loaded [/etc/hosts], raises MarkupError. The error reaches the outer handler and ends the loop. This is the freeze the function's docstring warns about: nothing drains the pipe any more, and the agent blocks once the pipe fills.
2. Bracketed words are deleted. [info] starting prints as starting, and deps: [pkg==1.0] prints as deps: .
3. Long lines block the event loop. Rendering markup for long lines is slow, and the reader can't drain while it runs: an 82,000-character line took 28.9 s here. This is also why test_large_line_within_the_limit_is_streamed_in_full times out on slower machines.

Fix

Print a rich.text.Text with the dim prefix and the line appended as plain text. The read loop, the limits and the error handling are unchanged. Child output loses Rich's automatic number/string highlighting; I think plain is the right default for raw logs.

Verification

I drove stream_process_output with an in-memory reader:

input	main	this branch
[INST] hi [/INST], loaded [/etc/hosts], [info] starting, deps: [pkg==1.0], after the markup	warns Output streaming for TEST stopped on MarkupError(...); nothing printed	all five lines printed verbatim
one 82,000-character line	28.89 s	0.01 s

• New tests in tests/lib/cli/test_run_handlers_streaming.py:
  • a child that prints markup-like lines and then floods the pipe still exits 0, because the reader kept draining;
  • every markup-like line appears verbatim.
• uv run pytest -n 0 tests/lib/cli/test_run_handlers_streaming.py: 7 passed in 1.5 s, including the 82k-character test.
• ruff check and pyright are clean.

Confidence Score: 5/5

This PR appears safe to merge; no actionable issues were found.

What we checked:

• Brackets stop agent output: The runner passes the decoded line into Text, and the new tests check both intact output and a child that finishes after flooding the pipe.

Summary

The CLI now prints agent output as plain text while keeping its prefix dim. Markup-like content stays visible, and the reader can keep draining output after those lines.

• Uses Rich text assembly to style only the prefix.
• Adds checks for intact markup-like lines and continued output draining.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Agent stdout] --> B[Read and decode line]
  B --> C[Build plain Text with dim prefix]
  C --> D[Print to console]
  D --> B

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