Add SelectQuery.First and Selective Relation Columns

[cungminh2710]

This PR introduces two key enhancements to Rain ORM:

1. SelectQuery.First(ctx, dest): A convenience method that executes the query with an implicit LIMIT 1 and scans the result into dest. It returns sql.ErrNoRows if no result is found, matching Drizzle's behavior.

2. Selective Relation Columns: Enhanced RelationConfig to support a Columns field, allowing users to specify which columns to load for a relation. The implementation automatically ensures that relationship mapping keys (source/target columns) are included in the underlying SQL query to preserve data mapping integrity.

Test coverage includes both unit tests for SQL generation and integration tests using SQLite.

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PR created automatically by Jules for task 9720018403007912893 started by @cungminh2710

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[greptile-apps]

Greptile Summary

This PR adds two features: a First(ctx, dest) convenience method on SelectQuery that applies an implicit LIMIT 1 before scanning, and selective relation column loading via a new Columns []schema.Expression field on RelationConfig, with auto-inclusion of the join-mapping key column (ensureTargetColumnSelected).

• First is a one-liner clone-and-delegate that correctly propagates sql.ErrNoRows for struct destinations through the existing scanRowsWithRelations path; integration tests cover both the success and no-rows cases.
• Selective relation columns work correctly for single-level relations: ensureTargetColumnSelected guards the target FK column so data mapping survives user-specified column subsets.
• Gap: ensureTargetColumnSelected only covers the current relation's TargetColumn; when a relation with Columns specified is also a parent of nested child relations, the child's SourceColumn (e.g. posts.id for a comments child) is not auto-added to the query, causing nested relation loading to silently return empty or zero-keyed results.

Confidence Score: 4/5

Safe to merge for all single-level selective relation usage; the nested relation + selective columns combination produces silent data loss and should be fixed or documented before production use.

The First method is a clean, minimal addition with correct ErrNoRows propagation. The selective-columns feature works correctly for the tested single-level case. However, when Columns is set on a relation that also has child relations loaded, the child relation's source column (e.g. the PK of the intermediate table) is not auto-included in the SQL, so all nested reads silently return empty or zero-keyed results with no error returned to the caller.

pkg/rain/relation_loading.go — specifically the loadRelatedRows and loadRelatedManyToManyRows functions where selective column enforcement happens without accounting for child relation source columns.

Important Files Changed

Filename	Overview
pkg/rain/query_select.go	Adds First(ctx, dest) convenience method that clones the query, applies LIMIT 1, and delegates to Scan. Implementation is clean and minimal.
pkg/rain/relation_loading.go	Adds RelationConfig.Columns selective loading with ensureTargetColumnSelected guard. The guard correctly covers single-level loading but omits auto-inclusion of child-relation SourceColumns, which causes silent empty/wrong nested results when Columns is set on a non-leaf relation.
pkg/rain/query_select_test.go	Adds TestSelectFirst which validates LIMIT 1 SQL generation; does not invoke First() directly but tests the underlying mechanism; reasonable given prior review feedback.
pkg/rain/sqlite_integration_test.go	Adds integration tests for selective relation columns (single-level, with FK field in dest struct) and for First including ErrNoRows validation; both tests are well-structured and cover the happy path and the no-rows case.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant Caller
    participant SelectQuery
    participant loadRelationNode
    participant loadRelatedRows
    participant ensureTargetColumnSelected
    participant DB

    Caller->>SelectQuery: First(ctx, dest)
    SelectQuery->>SelectQuery: clone().Limit(1)
    SelectQuery->>DB: Scan(ctx, dest)
    DB-->>SelectQuery: rows
    SelectQuery-->>Caller: nil or sql.ErrNoRows

    Caller->>SelectQuery: Scan + Relation(posts, Columns:[title])
    SelectQuery->>DB: "SELECT * FROM users"
    DB-->>SelectQuery: parent rows
    SelectQuery->>loadRelationNode: load posts node
    loadRelationNode->>loadRelatedRows: sourceKeys, Columns:[title]
    loadRelatedRows->>ensureTargetColumnSelected: ensure posts.user_id
    loadRelatedRows->>DB: SELECT title, user_id FROM posts WHERE user_id IN (...)
    DB-->>loadRelatedRows: "post rows (id=0 in struct)"
    loadRelatedRows-->>loadRelationNode: relatedRows
    Note over loadRelationNode: node.children: posts.id missing
    loadRelationNode->>DB: SELECT FROM comments WHERE post_id IN (0)
    DB-->>loadRelationNode: empty result - silent data loss

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sequenceDiagram
    participant Caller
    participant SelectQuery
    participant loadRelationNode
    participant loadRelatedRows
    participant ensureTargetColumnSelected
    participant DB

    Caller->>SelectQuery: First(ctx, dest)
    SelectQuery->>SelectQuery: clone().Limit(1)
    SelectQuery->>DB: Scan(ctx, dest)
    DB-->>SelectQuery: rows
    SelectQuery-->>Caller: nil or sql.ErrNoRows

    Caller->>SelectQuery: Scan + Relation(posts, Columns:[title])
    SelectQuery->>DB: "SELECT * FROM users"
    DB-->>SelectQuery: parent rows
    SelectQuery->>loadRelationNode: load posts node
    loadRelationNode->>loadRelatedRows: sourceKeys, Columns:[title]
    loadRelatedRows->>ensureTargetColumnSelected: ensure posts.user_id
    loadRelatedRows->>DB: SELECT title, user_id FROM posts WHERE user_id IN (...)
    DB-->>loadRelatedRows: "post rows (id=0 in struct)"
    loadRelatedRows-->>loadRelationNode: relatedRows
    Note over loadRelationNode: node.children: posts.id missing
    loadRelationNode->>DB: SELECT FROM comments WHERE post_id IN (0)
    DB-->>loadRelationNode: empty result - silent data loss

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