fix case insensitive lookup for custom text models

[CODING-DARSH]

Fixes #650

Checklist

• Have you followed the guidelines in the Contributing document?
• Have you checked that there are no existing open PRs for the same issue?
• Does the change pass tests locally?
• Have you added a regression test for the fix?

Summary

Custom models can be registered using one casing (e.g. Org/Model) and later instantiated using a different casing (e.g. org/model). TextEmbedding already resolves models case-insensitively, but CustomTextEmbedding later looked up the postprocessing configuration using the original user-provided model name, which could result in a KeyError.

This change uses the canonical resolved model name from self.model_description.model when retrieving the postprocessing configuration.

Changes

• Fix custom text model postprocessing lookup to use the canonical resolved model name.
• Add a regression test covering mixed-case registration and instantiation.

Reproduction

Before

1. Register a custom model as Org/Model.
2. Instantiate TextEmbedding("org/model").
3. Construction can fail with KeyError.

After

1. Register a custom model as Org/Model.
2. Instantiate TextEmbedding("org/model").
3. Construction succeeds and uses the correct postprocessing configuration.

Testing

• Added test_custom_text_model_lookup_is_case_insensitive.
• Verified with:

poetry run pytest tests/test_custom_models.py -k custom_text_model_lookup_is_case_insensitive -q

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR changes how CustomTextEmbedding retrieves postprocessing configuration during initialization. Instead of using the model_name constructor argument to look up pooling and normalization settings in POSTPROCESSING_MAPPING, the code now uses self.model_description.model. A new test validates that custom models registered with case-variant names correctly preserve their pooling and normalization settings when instantiated via TextEmbedding.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly and concisely describes the main fix: resolving case-insensitive lookup for custom text models, which aligns with the core change in CustomTextEmbedding.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description accurately matches the code and test changes for case-insensitive custom text model postprocessing lookup.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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