Fixed #36632 -- Prevented AlterIndexTogether from dropping overlapping indexes.

[nzioker]

Trac ticket number

Fixed #36632

Branch description

Problem

AlterIndexTogether in historical migrations was dropping overlapping indexes when changing from [('a', 'b')] to [('a', 'b'), ('a', 'b', 'c')].

Root Cause

• alter_index_together used news.difference(olds) which only created new indexes
• _model_indexes_sql didn't include index_together indexes during SQLite table remakes

Solution

• Changed alter_index_together to iterate over all target indexes (news instead of news.difference(olds))
• Added historical index_together support to _model_indexes_sql
• Added regression test

Testing

• Original reproduction case now works
• All migration tests pass
• No regressions introduced

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.

[dnit]

test fails for

AssertionError: False is not true : Index on (a, b) was not found. Indexes: {'testapp_mymodel_a_b_221151a1_idx': 'CREATE INDEX testapp_mymodel_a_b_221151a1_idx ON public.testapp_mymodel USING btree (a, b)', 'testapp_mymodel_a_b_c_4f1d0c1a_idx': 'CREATE INDEX testapp_mymodel_a_b_c_4f1d0c1a_idx ON public.testapp_mymodel USING btree (a, b, c)'}