feat(memory): add session and user memory deletion APIs

[ArulJerald]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Memory has no removal path: delete_session() leaves what add_session_to_memory() copied, and BaseMemoryService exposes no way to remove it #7109

Problem:
Calling BaseSessionService.delete_session() removes session records from the session store, but leaves behind memories copied or synthesized into BaseMemoryService via add_session_to_memory() or add_events_to_memory(). Furthermore, BaseMemoryService offered no removal or purge API, making it impossible to clean up session memory or comply with user-level data retention / right-to-be-forgotten requirements.

Solution:

1. Added delete_session_memory(*, app_name, user_id, session_id) and delete_user_memory(*, app_name, user_id) to BaseMemoryService with default implementations raising NotImplementedError.
2. Implemented thread-safe session and user memory deletion in InMemoryMemoryService under self._lock, safely cleaning up empty user buckets.
3. Added delete_session_memory(session_id=None) (defaulting to the current session) and delete_user_memory() convenience helpers on Context.
4. Documented memory retention semantics on BaseSessionService.delete_session() and added a Lifecycle & Removal guide in docs/guides/memory/memory_service/index.md.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Summary of passed pytest results:

tests/unittests/memory/test_in_memory_memory_service.py: 45 passed
tests/unittests/agents/test_context.py: 44 passed
Total: 89 passed in 1.78s

Tests added:

• test_delete_session_memory: Verifies session deletion removes matching session memories while leaving other sessions intact.
• test_delete_session_memory_cleans_up_empty_user_bucket: Verifies user entry cleanup when the last session memory is purged.
• test_delete_session_memory_nonexistent_is_noop: Verifies idempotent no-op deletion.
• test_delete_user_memory: Verifies purging all memories for a user across multiple sessions.
• test_delete_user_memory_nonexistent_is_noop: Verifies idempotent no-op on missing user.
• test_base_memory_service_default_deletion_methods: Verifies NotImplementedError raised on unimplemented backends.
• test_delete_memory_is_thread_safe_against_concurrent_searches: Verifies thread safety under concurrent reader and deleter threads.
• test_delete_session_memory_default_current_session, test_delete_session_memory_explicit_session_id, test_delete_user_memory_success, and no-service error cases in test_context.py.

Checklist

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.