Python: fix(core): give each file memory its own description sidecar - #8621

Motivation & Context

FileMemoryProvider named the description sidecar by replacing the file's extension, so notes.md and notes.json both mapped to notes_description.md. Writing one of them without a description deleted the other's description, writing one with a description made file_memory_ls and memories.md show it for both, and file_memory_delete on a file that did not exist still removed the sidecar. Repro and output are in the linked issue.

Description & Review Guide

• What are the major changes?
  • _description_file_name now appends the suffix to the whole name (notes.md → notes.md_description.md), so files that differ only by extension no longer share a sidecar. _is_internal_file is unchanged; the new names still end in _description.md and stay hidden.
  • _legacy_description_file_name keeps the old extension-replacing name. _read_description (used by file_memory_ls and _rebuild_index) prefers the current sidecar and falls back to the legacy one, so descriptions written by earlier versions keep showing up.
  • _delete_description removes the current sidecar and removes the legacy one only when no other remaining memory file still maps to it (_legacy_sidecar_is_shared), since under the old scheme that sidecar may belong to a sibling.
  • file_memory_delete only touches sidecars and rebuilds the index when the file was actually deleted; a "not found" delete is now side-effect free.
• What is the impact of these changes?
  • New descriptions are written under the new name. Existing stores keep working: legacy sidecars are read until a new description replaces them and are cleaned up when their last owner is deleted.
  • The .NET FileMemoryProvider (GetDescriptionFileName) uses the same extension-replacing scheme and has the same collision; this PR only changes Python. Happy to do the .NET side as a follow-up if you want the layouts to match.
• What do you want reviewers to focus on?
  • Whether keeping the legacy read/cleanup path is wanted, or whether you would rather drop it and accept that pre-1.20 descriptions are no longer surfaced.

Tests: test_description_file_name_keeps_extension, test_legacy_description_file_name_replaces_extension, test_legacy_sidecar_is_shared_only_with_other_visible_files, test_files_differing_only_by_extension_keep_separate_descriptions (the three symptoms from the issue), test_legacy_description_sidecar_is_still_read_and_cleaned_up; existing sidecar tests updated to the new name. test_harness_file_memory.py: 39 passed. poe fmt, poe lint, poe typing -P core clean.

Related Issue

Fixes #8620

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

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