Python: move _ensure_message_ids higher in the chain to avoid duplicate message ids

[jorge-wonolo]

Motivation and Context

Fixes #5237

_ensure_message_ids assigns f"msg_{index}" where index comes from enumerate over the list it receives.
annotate_message_groups calls it indirectly via group_messages(messages[start_index:]) on a slice, so
enumerate restarts from 0 on every incremental call — producing IDs that collide with the preserved prefix.

The most visible failure is in SummarizationStrategy: _group_id_for derives group_id from message_id, so
colliding IDs collapse distinct groups into one.

Description

Move the _ensure_message_ids(messages) call from group_messages() (which operates on a slice) to the top of
annotate_message_groups() (which holds the full list). This ensures every message is indexed against the
complete conversation, producing stable, unique IDs across incremental calls.

Changes:

• _compaction.py: Remove _ensure_message_ids call from group_messages, add it at the top of
  annotate_message_groups before any slicing occurs.
• test_compaction.py: Add regression test
  test_annotate_message_groups_assigns_unique_ids_after_incremental_calls that simulates multi-turn incremental
  annotation and asserts all message_ids remain unique.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution
  Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

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