feat: add InsufficientQuotaError for insufficient_quota 429 responses

[crawfordxx]

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Changes being requested

This PR adds a new InsufficientQuotaError exception class that subclasses RateLimitError, enabling users to distinguish between temporary rate limits and quota exhaustion when handling 429 responses.

Currently, both rate-limiting and insufficient-quota errors raise RateLimitError, but they require very different handling:

• Rate limit: retry after a delay
• Insufficient quota: switch to a backup API key, alert the user, etc.

What changed

• src/openai/_exceptions.py: Added InsufficientQuotaError(RateLimitError) with status_code = 429
• src/openai/_client.py: Updated _make_status_error in both OpenAI and AsyncOpenAI to check if the error code is "insufficient_quota" before falling back to RateLimitError
• src/openai/__init__.py: Exported InsufficientQuotaError

Backward compatibility

InsufficientQuotaError extends RateLimitError, so existing except RateLimitError blocks will continue to catch both error types. Users who want to handle quota exhaustion differently can now catch InsufficientQuotaError specifically:

try:
    client.chat.completions.create(...)
except openai.InsufficientQuotaError:
    switch_to_backup_key()
except openai.RateLimitError:
    time.sleep(retry_delay)

Additional context & links

Closes #1671