Sweep rescue nil from remaining specs (+ remove_context_index if_exists:)

[yahonda]

Summary

Continues the cleanup begun in PR #2687 (which swept connection_spec.rb). Replaces the @conn.execute "DROP …" rescue nil and friends pattern across the remaining spec files. The bare rescue nil swallows every error — privilege failures, ORA-00972 identifier-too-long on 11g, syntax errors — and lets specs assert against half-built fixtures (case in point: PR #2686 spent days chasing an ORA-00972 that the catch-all had hidden).

Two commits

1. Support if_exists: on remove_context_index (3109cd8f) — lib + spec

ActiveRecord's remove_index accepts if_exists: true so callers can issue idempotent cleanup without rescuing themselves; the Oracle Enhanced remove_context_index helper had no such option, so spec cleanup paths used rescue nil. This commit adds the option:

remove_context_index :posts, name: "post_and_comments_index", if_exists: true

if_exists: is forwarded to drop_if_exists("INDEX", index_name, if_exists: …) (rescues only ORA-01418 "specified index does not exist"). The auxiliary trigger and procedure cleanup inside remove_context_index are also routed through drop_if_exists (always if_exists: true since they're conditionally created), replacing inline rescue nil in those private helpers.

The drop_ctx_preference PL/SQL call is left as-is — Oracle Text returns DRG-* codes, not ORA-*, and narrowing it cleanly requires a separate per-code audit.

2. Sweep rescue nil cleanup paths across remaining specs (9fedcdd8) — six spec files

File	Pattern → replacement
oracle_enhanced_data_types_spec.rb	DROP TABLE + DROP SEQUENCE → drop_table(name, if_exists: true) (default <table>_seq is cleaned up by drop_table itself)
oracle_enhanced/dbms_metadata_structure_dump_spec.rb	DROP PACKAGE → drop_if_exists("PACKAGE", name)
oracle_enhanced/schema_dumper_spec.rb	remove_foreign_key … rescue nil → remove_foreign_key …, if_exists: true; DROP MV → drop_if_exists("MATERIALIZED VIEW", …)
oracle_enhanced/schema_statements_spec.rb	DROP MV → drop_if_exists("MATERIALIZED VIEW", …)
oracle_enhanced/quoting_spec.rb	Object.send(:remove_const, "X") rescue nil → Object.send(:remove_const, "X") if Object.const_defined?("X")
oracle_enhanced/structure_dump_spec.rb	DROP VIEW/MV/SYN/PKG/FUNC/PROC/TYPE/TRIGGER → drop_if_exists("<TYPE>", name); ALTER TABLE … DROP CONSTRAINT/COLUMN lines simplified out (constraints/columns disappear with their parent tables)

Net behavior

The same expected misses are still tolerated, but unexpected failures (privileges, syntax, identifier limit, etc.) now propagate instead of being silently absorbed.

Test plan

• bundle exec rspec full suite — 748 examples, 0 failures, 12 pending (CRuby 4.0.3 / Oracle 23ai 23.26.1)
• bundle exec rubocop — clean
• Each touched file run individually green

Out of scope

A handful of rescue nil callsites remain in the spec suite that this PR does not touch:

• oracle_enhanced_adapter_spec.rb
• oracle_enhanced/primary_key_trigger_spec.rb (single SEQUENCE drop — minor)
• oracle_enhanced/schema_cache_spec.rb (5 drop_table … rescue nil — easy follow-up)
• oracle_enhanced/connection_spec.rb:50 (SystemObserver.remove_connection rescue nil — AR pool teardown, different pattern)
• oracle_enhanced/type/timestamp_spec.rb:30 (already has if_exists: true, just trailing rescue nil to drop)
• oracle_enhanced/procedures_spec.rb:319 ((TestEmployee.partial_updates = false) rescue nil — handled in a separate PR; the rationale is unrelated to fixture cleanup)

Will land as follow-ups so this PR stays focused on the original eight files identified in #2687's description.

Ref: PR #2686 (the original case where rescue nil hid an ORA-00972 identifier-too-long failure on 11g), PR #2687 (the connection_spec.rb half), PR #2688 (the drop_if_exists adapter helper).

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