feat: support nullable external id mapping

[marcelo-cjl]

Summary

• Add ExternalIdMap to centralize nullable and relayout id mapping.
• Wire mapped filtering and one-pass result id mapping across Knowhere index nodes, brute force, iterators, and vector retrieval paths.
• Add nullable external id map unit coverage.

Test plan

• git diff --check HEAD~1..HEAD
• Full compile/tests not rerun in this PR creation step.

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[mergify]

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[marcelo-cjl]

Closed in favor of fork-based PR #1673.