fix(llm): validate API key format in LiteLLMProvider to fail fast on malformed keys

[yk0007]

Fixes #1133

What

Adds _validate_api_key() to LiteLLMProvider.__init__() to reject malformed API keys at initialization instead of failing later at runtime with unclear auth errors.

Validation checks

1. Empty or whitespace-only keys
2. Keys containing ASCII control characters (0x00–0x1F, 0x7F)
   • covered examples: newline, carriage return, tab, NUL, DEL
3. Accidentally quoted keys, including whitespace-padded quotes

Impact

Applies to all providers that use LiteLLMProvider (OpenAI, Anthropic, Gemini, DeepSeek, Mistral, etc.).

Tests

Added 3 tests under TestLiteLLMProviderInit:

• test_init_with_malformed_key_whitespace
• test_init_with_malformed_key_control_chars
• test_init_with_malformed_key_quotes

Validation run:

• make check ✅
• make test ✅ (686 passed, 71 skipped)