input: Add text decoration collections (#2602)

## Summary

- add `TextDecoration` and `TextDecorationCollection` as a small,
presentation-only API for input decorations
- expose Rust-style `create_decorations_collection`, returning a
collection handle with `set`, `append`, `clear`, and `get_ranges`
- keep collection identity internal instead of exposing opaque IDs; the
handle uses a weak reference to its owning `InputState`
- use GPUI `HighlightStyle` and UTF-8 byte ranges, with Monaco
Decorations API links on the corresponding public APIs
- keep decoration storage outside `state.rs` and compose it into the
existing rendering paths
- include an interactive Input Story example with multiple background
and underline ranges

<img width="982" height="757" alt="image"
src="https://github.com/user-attachments/assets/694c95ab-a7a0-4208-8c42-8a2973b9e30b"
/>

This supersedes #2599. The earlier approach touched more input and
UTF-16 conversion logic than the underline/highlight requirement needed.
This PR intentionally keeps the implementation localized and preserves
the existing input/highlighter behavior.

For follow-up changes, please modify only what the concrete requirement
needs and keep diffs narrowly scoped; avoid changing unrelated core
input behavior.

Monaco is used as a reference for the ownership model and API
capabilities, while naming, types, and return values follow Rust and
GPUI conventions rather than copying the TypeScript API literally.

## Test Plan

- `cargo test -p gpui-component --lib` (325 passed)
- `cargo test -p gpui-component decoration --lib` (3 passed)
- `cargo check -p gpui-component-story`
- `git diff --check`

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Co-authored-by: Codex <codex@openai.com>