fix(tui): linkify complete bare URLs with tildes (#27088)

## Background

Bare URLs containing `~` in their path are currently only clickable up
to the tilde in the interactive TUI. For example, Codex renders the
visible text for:


`https://www.cs.tufts.edu/~nr/cs257/archive/olin-shivers/dissertation.pdf`

but the OSC 8 destination stops at `https://www.cs.tufts.edu/`. This
makes Cmd-click open the wrong location even though the terminal
recognizes the complete URL outside Codex.

Fixes #26774.

## Root Cause

The URL scanner already accepts `~`. The truncation happens earlier:
with strikethrough parsing enabled, `pulldown-cmark` splits this URL
into adjacent decoded `Event::Text` values around the tilde. The
Markdown renderer annotated each text event independently, so only the
first event still looked like a complete URL with a supported scheme.

The renderer now merges adjacent decoded text events before URL
annotation. It preserves the combined source range while retaining
parser-decoded contents, which avoids regressing entities such as
`&`.

## Changes

- Add a small iterator that merges adjacent decoded Markdown text events
and their source ranges.
- Apply it at the Markdown renderer boundary before hyperlink detection.
- Add regression coverage for the reported URL in prose, wrapped table
output, and entity-decoded URLs.

## How to Test

1. Run Codex with `just c`.
2. Ask the assistant to output this exact bare URL with no Markdown link
syntax:

`https://www.cs.tufts.edu/~nr/cs257/archive/olin-shivers/dissertation.pdf`
3. Hold Cmd and hover or click the URL.
4. Confirm the complete URL, including the suffix after `~`, is one
destination.
5. Repeat with the URL inside a Markdown table and confirm wrapped
portions retain the same complete destination.

Targeted tests:

- `just test -p codex-tui url_with_tilde`
- `just test -p codex-tui merged_text_events_preserve_entity_decoding`

The full `codex-tui` test run was also executed. Its only failures were
the two existing Guardian feature-flag tests:

-
`app::tests::update_feature_flags_disabling_guardian_clears_review_policy_and_restores_default`
-
`app::tests::update_feature_flags_disabling_guardian_clears_manual_review_policy_without_history`

codex-rs/tui/src/markdown_render.rs

@@ -39,6 +39,7 @@
 //! body rows, or even 3-char-wide columns cannot fit, body rows render as
 //! key/value records.
 
+use crate::markdown_text_merge::DecodedTextMerge;
 use crate::render::highlight::foreground_style_for_scopes;
 use crate::render::highlight::highlight_code_to_lines;
 use crate::render::line_utils::line_to_static;
@@ -322,7 +323,7 @@ pub(crate) fn render_markdown_lines_with_width_and_cwd(
     let mut options = Options::empty();
     options.insert(Options::ENABLE_STRIKETHROUGH);
     options.insert(Options::ENABLE_TABLES);
-    let parser = Parser::new_ext(input, options).into_offset_iter();
+    let parser = DecodedTextMerge::new(Parser::new_ext(input, options).into_offset_iter());
     let mut w = Writer::new(input, parser, width, cwd);
     w.run();
     w.text