…ies#57287)

When `read_file` returns a file outline instead of full contents, the
result is now wrapped in a plain fenced code block rather than tagging
it with the file's path.

Previously, the outline was wrapped in a fenced block tagged with the
file path (e.g. `` ```crates/agent/src/tools/read_file_tool.rs ``).
Because the tag contains a slash, the markdown parser routed it through
`CodeBlockKind::FencedSrc`, resolved the file's language by path, and
ran the language's tree-sitter parser against the outline on every
paint. The outline is structural (e.g. `fn foo [L10-20]`), not actual
source for the file's language, so the parse was both expensive and
produced incorrect highlighting.

Because GPUI rebuilds the visible element tree on every window paint,
anything that triggers a repaint (cursor blink in the focused message
editor, animations, the turn timer, etc.) would re-run the tree-sitter
parse on the entire outline, throttling the frame rate while the tool
call was expanded.

This change adds an `is_outline_response` flag in `ReadFileTool::run`
and, when set, passes an empty tag to `MarkdownCodeBlock` so the
renderer sees `CodeBlockKind::Fenced` (no language). Plain monospace
formatting is preserved; the path tag is still used for the non-outline
(full file) case.

Adds two regression tests: one asserting the outline path uses an
untagged fenced block, and one asserting the full-file path keeps the
path tag (so the next person fixing this doesn't accidentally strip the
tag everywhere).

Also includes a small markdown-rendering follow-up:
`MarkdownElementBuilder::push_text` was recomputing the base text style
(cloning `base_text_style` and walking the style stack) twice per
highlighted token. For a code block with hundreds of highlight tokens,
that's hundreds of redundant `TextStyle` clones per paint. The style
stack does not change while runs are being attributed, so the style is
now computed once outside the loop and reused.

Closes AI-234

Release Notes:

- Improved scrolling smoothness in the agent panel when a `read_file`
tool call with a large file outline is expanded.