fix(cache): validate cache_index schema collisions in worker dat…

[Hitanshi7556]

What this PR does
Adds schema validation in WorkerDataSource::try_new to reject source tables that already define cache_index.

Why
cache_index is appended by data-cache worker logic; collisions can lead to ambiguous schema behavior.

Scope

• Validation + error path in worker datasource
• Unit tests for collision / non-collision
• No projection/subset-scan changes (left for follow-up)

Testing

• make test-rust
• focused worker datasource tests

fixes #3174

[google-oss-prow]

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Pull request overview

This PR adds schema validation to prevent column name collisions when the data cache worker appends its cache_index column to source table schemas. The validation rejects source tables that already define a cache_index column, addressing the TODO comment that was previously in the code.

Changes:

• Added validation in WorkerDataSource::new to check for cache_index column collision before appending it to the schema
• Removed the TODO comment about validating name collisions
• Added two unit tests to verify the schema collision detection logic