feat: coalesce helper

[cungminh2710]

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[greptile-apps]

Greptile Summary

This PR introduces a typed COALESCE helper to the ORM, replacing the need to write raw SQL strings like Raw("COALESCE(...)") with a first-class schema.Coalesce(expr1, expr2, ...) expression that is validated and compiled by the query engine.

• Adds CoalesceExpr struct implementing the Expression interface with an .As() alias method, consistent with AggregateExpr and other expression types.
• Adds schema.Coalesce(...Expression) constructor that panics eagerly on fewer than two arguments or any nil argument.
• Adds a case schema.CoalesceExpr: branch in the compiler's writeExpressionInContext switch that re-validates at compile time (defensive against direct struct construction) and emits COALESCE(expr1, expr2, ...) with correct dialect-aware placeholders.
• Updates the ORM showdown benchmark to use the new typed helper.
• Comprehensive test coverage: happy-path unit tests, panic/arity tests, and a full SQL-output integration test for Postgres.

Confidence Score: 5/5

• Safe to merge — the implementation is clean, consistent with existing patterns, and well-tested with no apparent logic or correctness issues.
• All changes are additive, validation is applied both at construction time (panic) and compile time (error), tests cover happy-path, aliasing, arity panics, nil panics, and SQL output. No regressions to existing paths.
• No files require special attention.

Important Files Changed

Filename	Overview
pkg/schema/schema.go	Adds CoalesceExpr struct with isExpression()/As() methods and a validated Coalesce() constructor that panics on <2 args or nil expressions; consistent with existing aggregate helpers.
pkg/rain/query_compile.go	Adds CoalesceExpr case to the expression compiler switch; re-validates arity and nil elements at compile time to handle direct struct construction, then emits well-formed COALESCE(…) SQL.
pkg/rain/query_select_test.go	Adds an integration test for the new Coalesce helper in a Postgres SELECT, verifying correct SQL and bound args output.
pkg/schema/schema_internal_test.go	Adds unit tests for Coalesce() happy-path payload, isExpression() marker, and both panic conditions (arity and nil).
pkg/schema/schema_test.go	Adds public API tests for Coalesce() expression count and aliasing via .As().
pkg/rain/query_compile_internal_test.go	Adds a compile-layer test for the arity error returned when CoalesceExpr is constructed with fewer than two expressions directly.
benchmarks/ormshowdown/harness_test.go	Migrates one benchmark usage from Raw("COALESCE(…)") to the new typed Coalesce() helper.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["schema.Coalesce(exprs...)"] -->|"len < 2 or nil"| B["panic (eager validation)"]
    A -->|"valid"| C["CoalesceExpr{Exprs: exprs}"]
    C -->|".As(alias)"| D["AliasExpr{Expr: CoalesceExpr, Alias: alias}"]
    C --> E["query compiler\nwriteExpressionInContext"]
    D --> E
    E -->|"len < 2 or nil element"| F["return error (compile-time guard)"]
    E -->|"valid"| G["emit COALESCE(expr1, expr2, ...)"]
    G --> H["dialect placeholder substitution\n(? for MySQL, $N for Postgres)"]

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