refactor: make provider_id mandatory in vector DB registration to avoid ambiguity

[HabebNawatha]

What does this PR do?

This PR improves vector database registration by making provider_id a mandatory field, aligning with best practices across the codebase and ensuring consistent, explicit behavior.

Previously, if multiple vector_io providers were registered and no provider_id was provided, the system would arbitrarily pick the first provider or raise a less-clear error. This change eliminates ambiguity and ensures users always specify which provider to use.

Closes #2834

Test Plan

• Modified the register_vector_db method signature to require provider_id explicitly.
• Verified that API now returns a validation error when provider_id is missing.
• Confirmed behavior with multiple and single provider setups to ensure proper enforcement.

[leseb]

@HabebNawatha see unit test failures, thanks