fix(tui): render file links with literal spaces

[middleDuckAi]

Summary

• tolerate literal spaces in file:// local link targets
• keep local file links styled and rendered in both TUI markdown renderers
• add matching regression coverage in both codex-tui and codex-tui-app-server

Why

Issue openai#14688 reports that local file:// links with literal spaces degrade to plain text because Url::parse rejects the raw target. That makes valid local references render inconsistently compared with already-encoded file links.

This change adds a narrow space-encoding fallback for file:// parsing only, so the local-link renderer still shows the styled target path.

Testing

• cargo fmt --all
• cargo test -p codex-tui file_link_with_literal_spaces_uses_target_path
• cargo test -p codex-tui-app-server file_link_with_literal_spaces_uses_target_path

Notes

• The fix is intentionally narrow: only file:// parsing gets a space-encoding fallback.
• Plain path-like links and already-valid URLs keep the existing behavior.