## Objective

Make GPT-6 Astra selectable through the OpenAI API-key provider. The
built-in catalog omitted it, even though it was available through the
separately fetched ChatGPT subscription catalog.

## Solution

Add `gpt-6-astra` with its documented context/output limits, reasoning
efforts, image support, and Responses capabilities. Existing model
defaults remain unchanged.

Handle two API compatibility differences found during review: Astra
rejects `temperature`, and its Fast-mode responses return `service_tier:
"fast"` even when requested as `"priority"`. Both request converters now
omit Astra's temperature, and response decoding accepts the new spelling
while preserving legacy serialization. Two focused regressions cover
these failures without duplicating the catalog declaration.

References: [Astra
model](https://developers.openai.com/api/docs/models/gpt-6-astra),
[migration
guide](https://developers.openai.com/api/docs/guides/latest-model?model=gpt-6-astra),
[Fast mode](https://developers.openai.com/api/docs/guides/fast-mode).

## Testing

- `corgi test -p open_ai --lib`: 88 passed.
- `corgi check -p language_models --lib`: passed.
- `corgi fmt -p open_ai -p language_models -- --check` and `git diff
--check`: passed.
- Live API probes confirmed model availability, a successful Responses
function-tool call with parallel calls enabled, the `fast` response
spelling, and rejection of configured temperature.
- During development, Delta's existing API-key catalog test passed with
a temporary local dependency override and an Astra frontier expectation.
The override was removed; Delta's dependency bump is separate.
- Validation was on macOS. No interactive picker, browser, or Cloud
Runner workflow was exercised; image input, compaction, and Fast-mode
regional availability were checked against the docs rather than live
workflows.

## Before merge

The README contains the repository-required human-review confirmation
marker. The author must remove it after reviewing. Delta will need a
separate dependency bump after this change lands. Cache-preserving
reasoning-effort updates are outside this change.

Release Notes:

- Added GPT-6 Astra to the OpenAI API-key model picker.