BYOK model limits catalog is missing gpt-5.5

### Describe the bug

When using BYOK / a custom OpenAI-compatible provider with `gpt-5.5`, Copilot CLI treats the model as missing from the built-in limits catalog and falls back to default token limits.

This appears to be a catalog mismatch: `gpt-5.5` can be listed/used by the provider, but the BYOK limits catalog does not include an entry for it. As a result, context/compaction uses a much smaller default prompt budget unless users manually set `COPILOT_PROVIDER_MAX_PROMPT_TOKENS` and `COPILOT_PROVIDER_MAX_OUTPUT_TOKENS`.

### Affected version

GitHub Copilot CLI 1.0.40

### Steps to reproduce the behavior

Use an OpenAI-compatible BYOK provider that exposes `gpt-5.5`, then run Copilot CLI with the model selected but without manual token limit overrides:

```bash
export COPILOT_PROVIDER_TYPE=openai
export COPILOT_PROVIDER_BASE_URL=/v1
export COPILOT_PROVIDER_API_KEY=
export COPILOT_PROVIDER_MODEL_ID=gpt-5.5
export COPILOT_PROVIDER_WIRE_API=responses
unset COPILOT_PROVIDER_MAX_PROMPT_TOKENS
unset COPILOT_PROVIDER_MAX_OUTPUT_TOKENS

copilot --log-level debug -p 'Reply with exactly OK and nothing else.'
```

Observed debug log excerpts:

```text
[WARNING] Model "gpt-5.5" is not in the built-in catalog. Using defaults for: prompt tokens (COPILOT_PROVIDER_MAX_PROMPT_TOKENS), output tokens (COPILOT_PROVIDER_MAX_OUTPUT_TOKENS). Run `copilot help providers` for configuration details.
[INFO] Using custom provider: type=openai, wireApi=responses
[DEBUG] Using model: gpt-5.5
[DEBUG] Listed models: [gpt-5.5,gpt-5.5]
"max_prompt_tokens": 128000,
"max_context_window_tokens": 200000,
CompactionProcessor: Utilization 15.4% (19674/128000 tokens) below threshold 80%
```

If manual overrides are provided, they do take effect:

```bash
export COPILOT_PROVIDER_MAX_PROMPT_TOKENS=400000
export COPILOT_PROVIDER_MAX_OUTPUT_TOKENS=128000
```

Then the compaction log uses the manual prompt limit:

```text
CompactionProcessor: Utilization 4.9% (19666/400000 tokens) below threshold 80%
```

### Expected behavior

`gpt-5.5` should either:

1. be included in the BYOK built-in model limits catalog with appropriate prompt/context/output limits, or
2. have a documented way to inherit/resolve limits for this model without falling back to the generic defaults.

### Actual behavior

`gpt-5.5` is usable from the provider model list, but the BYOK model limits lookup misses it and falls back to defaults:

```text
max_prompt_tokens: 128000
max_context_window_tokens: 200000
```

### Additional context

The nearby GPT-5.x entries in the BYOK catalog appear to include models such as `gpt-5.4`, `gpt-5.4-mini`, and `gpt-5.4-nano` with larger limits. The issue seems specific to `gpt-5.5` being absent from the built-in BYOK limits catalog.