Remove obsolete custom time-partition column references #1829

Summary

Parseable no longer supports setting a custom time-partition column, as clarified by @parmesant in PR #1828. Remove obsolete custom time-partition references from the codebase instead of extending manifest pruning to support them.

Required changes

• Audit the codebase for custom time-partition configuration, metadata fields, query-planning branches, and related references.
• Remove obsolete custom time-partition parameters and branches from query planning and timestamp-bound extraction. Known references occur in src/query/stream_schema_provider.rs, including PartialTimeFilter::try_from_expr, extract_timestamp_bound, extract_primary_filter, and scan planning.
• Audit associated storage metadata handling, ingestion paths, tests, and documentation. Remove or update references that imply custom time-partition columns remain supported.
• Determine how existing serialized metadata containing custom time-partition fields should be handled. Preserve safe metadata loading or document any required migration.
• Preserve pruning and ordering behavior for the supported default timestamp column, p_timestamp. Keep filters on unrelated timestamp columns from being used as manifest time bounds.

Rationale

Retaining custom time-partition branches implies support for an unavailable feature and can lead to incorrect review recommendations or future changes that reintroduce unsupported behavior.

Acceptance criteria

• Active code and documentation no longer expose or depend on configurable custom time-partition columns.
• Any retained legacy metadata handling has an explicit compatibility rationale.
• Tests cover supported p_timestamp pruning and ensure that unrelated timestamp predicates are not used as manifest time bounds.
• Query filters on unrelated timestamp columns remain enforced for staging and synced data.

Context

Requested by @parmesant.

• PR: fix: manifest pruning #1828
• Review discussion: fix: manifest pruning #1828 (comment)