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 (1081fe8) with master (cbaea35)¹}

Summary

✅ 6 untouched
⏩ 28 skipped²

Footnotes

1. No successful run was found on master (4e4e5d7) during the generation of this report, so cbaea35 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 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. ↩

[Copilot]

Pull request overview

This PR fixes a case-sensitivity bug in GPT-5 model temperature validation that prevented users from using uppercase or mixed-case model names (common with Azure deployments). The fix ensures model name comparisons are case-insensitive for both temperature validation and tiktoken encoder selection.

Key changes:

• Model name comparisons now use .lower() before checking with startswith() for GPT-5 and o1 models
• Encoder selection logic refactored to use tuple-based startswith() (more Pythonic)
• Comprehensive test coverage added for various case combinations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
libs/partners/openai/langchain_openai/chat_models/base.py	Updated validate_temperature() and _get_encoding_model() methods to perform case-insensitive model name comparisons
libs/partners/openai/tests/unit_tests/chat_models/test_base.py	Added parametrized test covering uppercase, mixed-case, and lowercase GPT-5 model names for both chat and non-chat variants