fix: avoid false positive AmbiguousCatalogMatchError for case-only differences

[chinar-amrutkar]

Resolves #11776

Problem

When a database has tables that differ only by case (e.g., 'Subject' and 'subject'), dbt docs generate incorrectly raises an error because the CatalogKey lowercases both the schema and table name, causing them to collide.

Solution

This fix:

1. Adds a check to detect case-only differences between existing and new table entries
2. When such a case-only difference is detected, skip adding the duplicate to avoid false positive errors
3. Keeps backward compatibility by adding optional case_sensitive parameter to mapping_key and get_unique_id_mapping for future use

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

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