docs: explain budget denials during LLM retries and fallback - #836

Running Vale across on the following files affected by this PR:

- sdk/guides/llm-error-handling.mdx
- sdk/guides/llm-fallback.mdx

Attempting to initialize Vale linter...
No customer vale configuration detected. Using Mintlify's default configuration.
Attempting to run Vale spellcheck...
Vale spellcheck completed successfully.

sdk/guides/llm-error-handling.mdx:
	- 6:159     Did you really mean 'Groq'?
	- 12:171    Did you really mean 'backoff'?

sdk/guides/llm-fallback.mdx:
	- 3:42      Did you really mean 'LLMs'?
	- 10:174    Did you really mean 'LLMs'?
	- 14:65     Did you really mean 'LLMs'?
	- 17:6      Did you really mean 'pydantic'?
	- 25:1      Did you really mean 'fallback_llm'?
	- 30:26     Did you really mean 'fallback_llm'?
	- 33:1      Did you really mean 'primary_llm'?
	- 216:89    Did you really mean 'multimodal'?


Found 10 suggested changes across 2 files in 460.08ms