fix(openai): make GPT-5 temperature validation case-insensitive

[Abhinavexists]

Fixed a bug where GPT-5 temperature validation was case-sensitive, causing issues when users
specified Azure deployment names or model names in uppercase (e.g., "GPT-5-2025-01-01", "GPT-5-NANO"). The validation now correctly handles model names regardless of case.

Changes made:

• Updated validate_temperature() method in BaseChatOpenAI to perform case-insensitive
  model name comparisons
• Updated _get_encoding_model() method to use case-insensitive checks for tiktoken encoder
  selection
• Added comprehensive unit tests to verify case-insensitive behavior with various case
  combinations

Issue: Fixes #34003

Dependencies: None

Test Coverage:

• All existing tests pass
• New test test_gpt_5_temperature_case_insensitive covers uppercase, lowercase, and
  mixed-case model names
• Tests verify both non-chat GPT-5 models (temperature removed) and chat models (temperature
  preserved)
• Lint and format checks pass (make lint, make format)

[codspeed-hq]

CodSpeed Performance Report

Merging #34012 will not alter performance

_{Comparing Abhinavexists:openai-case-sensitive (c24378f) with master (32bbe99)}

Summary

✅ 6 untouched
⏩ 28 skipped¹

Footnotes

1. 28 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩