Add composite primary key support

[yahonda]

Summary

Adds full composite primary key support to the Oracle Enhanced adapter, tracking the native Active Record CPK feature added in Rails 7.1 (rails/rails#48541). Closes #2648 and supersedes the partial fix in #2530.

Until now, oracle-enhanced treated composite PKs as unsupported:

• pk_and_sequence_for emitted a runtime warning (visible in every test run that touched a composite-PK table such as test_authors_test_books) and returned nil.
• primary_key returned nil for composite-PK tables.
• The insert / RETURNING path was single-column-only.
• empty_insert_statement_value raised NotImplementedError for an Array primary key.

Adapter changes

• pk_and_sequence_for — drop the "Composite primary key is ignored" warning. The method still returns nil for composite PKs (a single sequence cannot model a composite key); callers fall back to primary_keys.
• primary_key — removed; Rails' abstract default (pk = pk.first unless pk.size > 1) already returns an Array for composite PKs and a String / nil for single-column / no-PK tables.
• has_primary_key? — uses primary_keys directly so it returns true for composite-PK tables.
• prefetch_primary_key_from_dictionary — short-circuits composite PKs to false using a cardinality check (pks.size > 1), not composite_primary_key?(pks) — the predicate is Array?-based and primary_keys always returns an Array (including ["id"] for single-column PKs), so an Array? check would have wrongly disabled prefetch for every PK table when the schema cache is cold. The schema-cache fast path keeps using the existing composite_primary_key?(pks) predicate, where schema_cache.primary_keys returns a String for single-column PKs.
• empty_insert_statement_value — accepts an Array primary key and produces a multi-column (c1, c2, ...) VALUES (DEFAULT, DEFAULT, ...) form.
• columns_for_returning_clause — accepts an Array pk so multi-column RETURNING is generated. The "all-defaults" rejection-skip regex is widened from /VALUES \(DEFAULT\)/ to /VALUES \(DEFAULT(?:, DEFAULT)*\)/ so composite PKs whose columns all default still get RETURNING for the database-generated values rather than being dropped by the column-list rejection logic.
• write_lobs — gets a TODO documenting that its scalar-PK assumption breaks for CPK + LOB models. Out of scope for this PR; tracked separately.

Validations stay as-is: identity: true and primary_key_trigger: true combined with a composite primary key still raise ArgumentError.

Tests

• New spec/.../composite_primary_key_spec.rb ports the adapter-relevant cases from Rails' own activerecord/test/cases/primary_keys_test.rb so they run against Oracle. Coverage:
  • introspection (primary_keys, primary_key, has_primary_key?, pk_and_sequence_for returning nil and emitting no warning)
  • schema cache caching and invalidation of the composite PK column array
  • model contract (composite_primary_key?, id= array assignment, id= with non-array raising TypeError, id_was, id?, to_key, primary_key_values_present?)
  • schema dump round-trip (in-order and out-of-order composite PKs)
  • prefetch_primary_key? — false for composite, true for single PK, including a cold-cache regression that exercises the dictionary fallback
  • RETURNING — both the "user supplies all PK columns" and the "all PK columns defaulted" path on a CPK table with column defaults
  • find / find with multiple keys (preserves request order) / wrapped-array find / class-level destroy / predicate-builder where-by-PK-columns
  • mixed-type CPK (String + Integer) round-trip and find/destroy
• identity_primary_key_spec.rb's empty_insert_statement_value composite assertion is updated to expect the new SQL fragment instead of NotImplementedError.

Test plan

• bundle exec rspec spec/active_record/connection_adapters/oracle_enhanced/composite_primary_key_spec.rb — all examples pass
• Full local suite on Oracle 23ai (FREEPDB1) — 804 examples, 0 failures, 12 pending (all pre-existing skips)
• Local suite output no longer contains WARNING: Active Record does not support composite primary key.
• bundle exec rubocop — clean
• Fork CI on Ruby 3.3 / 3.4 / 4.0 / JRuby 10.1.0.0 + RuboCop — all green on the squashed head
• Upstream CI on the full matrix (does not auto-run on fork PRs; needs maintainer approval)