feat(redshift): make partition vals optional for redshift external tables

[nitish-raj]

Description & motivation

Redshift Spectrum doesn’t require partitions to be registered at table-create time. Many users register partitions later via Glue/Athena/event-driven flows. Forcing vals leads to brittle date math or references to partition values that do not exist, and it causes failures when vals are omitted.

Changes

• Make partition vals optional for Redshift during stage_external_sources .
• If every partition has vals, generate ALTER TABLE … ADD PARTITION (unchanged).
• If any partition lacks vals, skip only the partition-registration DDL; table creation and partition columns remain unchanged.

User-visible behavior

• Before: Missing vals → error ('dict object' has no attribute 'vals').
• Now: Missing vals → succeeds; no partition DDL is emitted. Existing partitions remain unchanged.

Checklist

• I have verified that these changes work locally
• I have updated the README.md (if applicable)
• I have added an integration test for my fix/feature (if applicable)

[nitish-raj]

@jeremyyeo Could you please take a look at this PR. It is quite important and fixes a major blocker for external tables setup in Redshift.

Also looks like the snowflake integration test is failing due to access issue, also highlighted in another PR.

Thank you.

[github-actions]

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