Add configurable inline completion debounce timeout (#61568)

# Objective

Add configurable inline completion debounce timeout
Fixes/implements #23159

## Solution

I initially wanted to make a global setting for this, but it would
conflict with hardcoded debounces in codestral (150ms) and copilot
(75ms) which I assume are there for a reason.

So I ended up using the same mechanism used for the hardcoded debounce
in Codestral (`DEBOUNCE_TIMEOUT`) and Copilot
(`COPILOT_DEBOUNCE_TIMEOUT`)
and made it accessible and configurable for all providers.


Also fixed a bug with `DelayMs` `Display` trait adding "ms" into the
input field which then fails to parse something like "150ms" as a `u64`
by implementing `FromStr` which strips the "ms" suffix if present.

So now both "1000" and "1000ms" are parsed correctly and apply.

If the parsing fix is not relevant enough I can open a separate issue +
PR for that (and the inconsistent use and therefore display of
`Option<u64>` vs `Option<DelayMs>` in other ms input fields).

## Testing

#### Did you test these changes? If so, how?

Added a separate test which passes
`test_refresh_prediction_from_buffer_honors_debounce_duration`

Manually tested with openapi compatible prediction

All other tests in affected crates pass (`cargo test -p settings_content
-p settings_ui -p editor -p edit_prediction -p language
`)

#### How can other people (reviewers) test your changes? Is there
anything specific they need to know?

Open provider settings and adjust debounce, then see how long it takes
for a prediction to render.

#### If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?

Tested on Fedora 43 KDE, but it shouldn't matter as none of the affected
code is platform specific.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase


https://github.com/user-attachments/assets/14efa628-765e-4c2a-ac44-01aaa3657097

---

Release Notes:

- Added configurable inline completion debounce timeout, fixes #23159

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>

docs/src/reference/all-settings.md

@@ -685,6 +685,29 @@ For the case of "open", regular selection behavior can be achieved by holding `a
 
 List of `string` values.
 
+### Prediction Debounce
+
+- Description: How long Zed waits after you stop typing before automatically requesting an edit prediction.
+- Setting: `edit_predictions.<provider>.prediction_debounce`
+- Default: `75` for GitHub Copilot, `150` for Codestral, and `0` for Zed, Mercury, Ollama, and OpenAI-compatible APIs.
+
+**Options**
+
+Non-negative integer values representing milliseconds. Set this to `0` to disable the additional delay. Configure the value under the settings object for the selected provider:
+
+```json [settings]
+{
+  "edit_predictions": {
+    "provider": "open_ai_compatible_api",
+    "open_ai_compatible_api": {
+      "prediction_debounce": 500
+    }
+  }
+}
+```
+
+See [Configuring the Prediction Debounce](../ai/edit-prediction.md#configuring-the-prediction-debounce) for more information.
+
 ## Edit Predictions Disabled in
 
 - Description: A list of language scopes in which edit predictions should be disabled.