Configurable LSP `workspace/configuration` and `didChangeConfiguration` Support in Copilot CLI

### Describe the feature or problem you'd like to solve

Add support for configuring arbitrary Language Server Protocol (LSP) configuration payloads in Copilot CLI.

### Proposed solution

Currently, Copilot CLI can launch LSP servers, but there is no way to control what is sent through the following LSP methods:

* `workspace/didChangeConfiguration`
* `workspace/configuration`

This makes many language servers partially unusable because critical settings cannot be provided to the server.

## Problem

Many LSP servers rely heavily on runtime configuration delivered through LSP configuration requests instead of command-line arguments or environment variables.

For example, `intelephense` requires configuration for:

* excluded file patterns
* storage/indexing behavior
* diagnostics
* stubs
* file size limits

Without support for configuration payloads:

* large monorepos become slow or unusable
* excluded/vendor/generated files cannot be filtered
* indexing behavior cannot be tuned
* workspace-specific settings cannot be applied

At the moment, Copilot CLI appears to start the server successfully but provides no mechanism for forwarding user-defined configuration objects to the LSP server. The option `initializationOptions` does not solve this problem either because many LSP servers - including Intelephense - expect runtime settings to be delivered specifically through the standard LSP configuration flow (`workspace/didChangeConfiguration` and `workspace/configuration`).

## Proposed Solution

Allow arbitrary configuration blocks to be defined per language server and forwarded via:

* `workspace/didChangeConfiguration`
* responses to `workspace/configuration`

Example configuration:

```json
{
"php": {
"command": "intelephense",
"args": [
"--stdio"
],
"fileExtensions": {
".php": "php"
},
"configuration": {
"intelephense": {
"files": {
"exclude": [
"**/vendor/**",
"**/var/cache/**",
"**/node_modules/**"
]
},
"environment": {
"phpVersion": "8.5"
},
"diagnostics": {
"undefinedTypes": false
}
}
}
}
}
```

## Expected Behavior

Copilot CLI should:

1. Send the provided configuration object through `workspace/didChangeConfiguration`
2. Respond to `workspace/configuration` requests using the configured values
3. Support arbitrary nested JSON structures

### Example prompts or workflows

_No response_

### Additional context

_No response_

Das Issue identifiziert die LSP-Methoden workspace/didChangeConfiguration und workspace/configuration als relevante Einstiegspunkte; beginne damit nachzuverfolgen, wie Copilot CLI LSP-Server startet und diese Nachrichten weiterleitet. Als abgeschlossen gilt die Arbeit, wenn jeder Sprachserver über beide Methoden beliebig verschachtelte Konfigurationen empfangen kann, einschließlich der beispielhaften Intelephense-Einstellungen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.