fix: add missing PermissionDeniedError to litellm exceptions list

[paipeline]

Problem

LiteLLMExceptions.__init__() crashes with a ValueError when litellm includes exception classes not present in aider's EXCEPTIONS list:

ValueError: PermissionDeniedError is in litellm but not in aider's exceptions list

This is the same class of bug reported in #4829. The _load() method iterates dir(litellm) and raises ValueError for any *Error class that is a BaseException subclass but missing from aider's hardcoded list.

Fix

• Added PermissionDeniedError to the EXCEPTIONS list as non-retryable (HTTP 403 — the user needs to fix their API key permissions, retrying won't help)
• Added a descriptive message: "The API provider denied access. Check your API key permissions."
• Added a test for the new exception

Testing

All 7 tests in tests/basic/test_exceptions.py pass, including the new test_permission_denied_error test.

Fixes #4829

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