feat!: rename bm25 APIs to paradedb and always use the paradedb access method

[rebasedming]

What

Implements paradedb/paradedb#5706 as a breaking rename — no configurability, no fallbacks:

• add_bm25_index/remove_bm25_index/reindex_bm25 → add_paradedb_index/remove_paradedb_index/reindex_paradedb_index (old names removed) across connection helpers, migration DSL, and CommandRecorder
• DDL is a literal USING paradedb; schema dumps emit add_paradedb_index with no AM option; no pg_am probing anywhere

Requires pg_search 0.25.0+; released images fail at index creation as expected.

Tests

Against a pg_search 0.25.0 dev server: 355 unit / 287 integration examples, 0 failures, on Rails 8.1 and 7.2. rubocop --lint and api-coverage clean.

[codecov]

Codecov Report

❌ Patch coverage is 97.36842% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.48%. Comparing base (02f6f91) to head (752a2c1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/parade_db/migration_helpers.rb	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   90.37%   90.48%   +0.11%     
==========================================
  Files          20       20              
  Lines        2411     2439      +28     
  Branches      774      779       +5     
==========================================
+ Hits         2179     2207      +28     
+ Misses        232      190      -42     
- Partials        0       42      +42     

Flag	Coverage Δ
rails-paradedb	90.48% <97.36%> (+0.11%)	⬆️
rb3.2	90.76% <97.36%> (+0.10%)	⬆️
rb3.3	90.76% <97.36%> (+0.10%)	⬆️
rb3.4	90.48% <97.36%> (+0.11%)	⬆️
rb4.0	82.12% <97.36%> (+0.24%)	⬆️
rl7.2	90.48% <97.36%> (+0.11%)	⬆️
rl8.1	82.12% <97.36%> (+0.24%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
lib/parade_db/index.rb	90.06% <100.00%> (+0.70%)	⬆️
lib/parade_db/model.rb	83.22% <100.00%> (ø)
lib/parade_db/migration_helpers.rb	85.06% <95.83%> (+0.46%)	⬆️

... and 4 files with indirect coverage changes

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