feat: AlterSchema, dropPredicate, and embedded DropAttr

[mlwelles]

Adds raw schema-DDL primitives that complement UpdateSchema's object-template inference.

What

• Client.AlterSchema(ctx, schema string) — applies a raw DQL schema string directly, giving full control over predicate types, indexes, and directives. Useful for schema migrations that declare predicates no Go type models yet.
• Engine.dropPredicate — deletes a single predicate (and its data) from the embedded engine via posting.DeletePredicate.
• embedded_client.go — routes an Alter carrying DropAttr to dropPredicate, so the embedded (file://) path matches a remote Dgraph cluster's DropAttr semantics.

Tests

TestDropPredicateEmbedded runs the full declare → insert → confirm-present → DropAttr → confirm-gone cycle against the embedded engine (and against a live cluster when MODUSGRAPH_TEST_ADDR is set).

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Summary by cubic

Add raw schema DDL support via AlterSchema and implement DropAttr handling for the embedded engine to match remote Dgraph behavior. This enables precise schema migrations and safe single-predicate drops across embedded and remote modes.

• New Features
  • Client.AlterSchema(ctx, schema) applies raw Dgraph schema directly, bypassing UpdateSchema inference (full control over types, indexes, directives).
  • Embedded Alter with DropAttr deletes a predicate and its data by routing to engine dropPredicate, mirroring cluster semantics.
  • Engine adds dropPredicate using posting.DeletePredicate.

^{Written for commit bd16559. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}