test(ai-config): cover auth-source, api-key caching, context, clear-buffer

The big-module coverage push starts with ai-config (was 53/191, 27.7%). New test file covers seven helpers that don't depend on a live gptel install:

- `cj/auth-source-secret`: returns string secrets, funcall's function secrets, errors on missing.
- `cj/anthropic-api-key` / `cj/openai-api-key`: cache the result so subsequent calls don't re-hit auth-source.
- `cj/gptel--add-file-to-context`: adds existing files, skips nil and missing paths.
- `cj/gptel-clear-buffer`: erases in a gptel-org buffer and reinserts the fresh prefix; no-ops + messages elsewhere.
- `cj/gptel-context-clear`: three cond branches (`remove-all`, `clear`, alist-fallback) with appropriate `fboundp` stubs.
- `cj/gptel-insert-model-heading`: inserts at the response-begin position with the backend/model heading.

External primitives (`auth-source-search`, `gptel-add-file`, `gptel-context-remove-all`, etc.) are stubbed. The fallback test declares `gptel-context--alist` as a top-level `defvar` so `setq` inside the function reaches the dynamic binding under lexical scoping.

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