feat: sort backend selection by MRU order with history persistence

[tninja]

This pull request implements backend selection ordering based on the user's most recently used (MRU) history, resolving the issue where the candidate order in C-c a s remained static.

Approach

1. MRU History Persistence: Added a history file ai-code-backends-history.el under the user-emacs-directory to serialize the list of recently selected AI backends.
2. Robust Loading: Implemented checks to gracefully discard and automatically delete the history file if it gets corrupted or contains malformed data.
3. Smart Reordering: Reordered candidate choices in ai-code-select-backend such that the active effective backend remains first, followed by valid historical backend keys in MRU order, and lastly any remaining available backends. This ensures all backends stay accessible even if the history is partial or contains outdated keys.

Verification

• Added comprehensive ERT unit tests in test_ai-code-backends.el to verify MRU list sorting, corrupted file automatic deletion, and partial history handling.
• Ran the entire test suite successfully to ensure all tests continue to pass and no regressions were introduced.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Validate history shape before returning it

If ai-code-backends-history.el contains valid Lisp that is not a proper list of backend symbols, such as a truncated atom or dotted list, this read returns it as-is instead of treating it as corrupted. The next C-c a s or backend switch then iterates/removes over that malformed value and can signal wrong-type-argument, leaving the selector broken until the user manually deletes the history file; the loader should reject and delete malformed-but-readable history too.

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