[#190]: Add upsert support via onConflict argument

[matt-metivier]

What

Implements upsert support via onConflict argument on insert mutations.
Closes #190

Why

This enables upsert operations (INSERT ... ON CONFLICT DO UPDATE) through the GraphQL API, which is a common requirement for applications that need idempotent writes.

Changes

• SQL Context: Load index names from pg_class to identify constraints
• SQL Types: Add on_conflict_indexes() and has_upsert_support() methods to Table
• GraphQL Types:
  • New {Table}OnConflict input type with constraint, updateColumns, and filter fields
  • New {Table}Constraint enum (unique index names)
  • New {Table}UpdateColumn enum (updatable column names)
  • Conditionally expose onConflict arg only on tables with valid constraints
• Builder: Parse onConflict argument and map GraphQL column names back to Column objects
• Transpile: Generate ON CONFLICT ON CONSTRAINT ... DO UPDATE SET ... WHERE ...

Example

mutation {
  insertIntoAccountCollection(
    objects: [{ id: 1, email: "new@example.com", name: "Updated" }]
    onConflict: {
      constraint: account_pkey
      updateColumns: [email, name]
      filter: { status: { eq: "active" } }
    }
  ) {
    affectedCount
    records { id email name }
  }
}

Notes

• Tables with serial/generated primary keys will not expose onConflict (constraint columns must be insertable)
• Empty updateColumns results in DO NOTHING
• Based on WIP - OnConflict support for Insert mutations #503 but reimplemented on current master without unrelated changes

Tests

Added tests in test/sql/mutation_insert_on_conflict.sql

[matt-metivier]

Whenever you've time @olirice, I tried to match & build on what you had in #503