entc/gen: bake Schema into generated code and allow same table names across schemas

[dannyota]

Fixes #2330

When two PostgreSQL schemas have tables with the same name (e.g., global.templates and lead.templates), code generation fails because Tables() rejects same table names across schemas and the template generates duplicate Go identifiers.

Changes

entc/gen/graph.go

• Use schema-qualified keys ("schema.table") in the tables map to allow same table names in different schemas
• Use nodeT map (*Type → *schema.Table) for FK lookups instead of bare table name lookups
• Add computeTableIdents() to generate unique Go identifiers: pascal(name) by default, pascal(schema) + "_" + pascal(name) on collision
• Add TableIdent(*schema.Table) string method on Graph, used by the schema template

entc/gen/template/migrate/schema.tmpl

• Replace all 6 pascal $t.Name / pascal $fk.RefTable.Name usages with $.TableIdent
• Emit the Schema field in generated table structs when set

dialect/sql/schema/schema.go

• Fix CopyTables to preserve Schema, Comment, View, Pos fields (previously silently dropped)
• Use schema-qualified keys in byName map and FK ref lookup

Backward Compatibility

Scenario	Behavior
No schema annotation	No Schema field emitted, identifiers unchanged. Generated code identical to before.
Single table per name	Go identifiers unchanged (e.g., UsersTable).
Same name, different schemas	New case — previously errored. Gets schema prefix (e.g., Global_TemplatesTable).
Manual Table.Schema overrides	Still work. If Schema is baked in, the manual set is a no-op.
CopyTables	Now correctly preserves Schema field (was silently dropped).

Testing

• Unit tests: TestMultiSchemaSameTableName, TestSingleSchemaTableIdent, TestDuplicateTableNameSameSchema, TestCopyTablesMultiSchema
• Integration: regenerated multischema/ent/migrate/schema.go with Schema baked in, verified against MySQL 8
• Regression: all integration tests pass (SQLite, MySQL 5.6/5.7/8, MariaDB, Postgres 10-17)