feat(memory): add DatabaseMemoryService with SQL backend and scratchpad

Adds a durable, RDBMS-backed memory service that works with any
SQLAlchemy-supported database (SQLite, PostgreSQL, MySQL, MariaDB)
as an alternative to the volatile InMemoryMemoryService.

Key additions:
- DatabaseMemoryService: implements BaseMemoryService with lazy table
  creation, idempotent session ingest, and delta event ingestion
- MemorySearchBackend ABC + KeywordSearchBackend: LIKE/ILIKE search
  with AND-first → OR-fallback tokenization strategy
- Scratchpad KV store and append-only log for intermediate agent state
- Four agent-callable BaseTool subclasses: scratchpad_get_tool,
  scratchpad_set_tool, scratchpad_append_log_tool, scratchpad_get_log_tool
- 38 unit tests covering all methods, tool happy-paths, wrong-service
  errors, multi-user isolation, and session scoping

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Raman369AI

.gitignore

@@ -99,6 +99,9 @@ Thumbs.db
 *.tmp
 *.temp
 
+# Agent handoff / session notes (not for version control)
+AGENT_HANDOFF.md
+
 # AI Coding Tools - Project-specific configs
 # Developers should symlink or copy AGENTS.md and add their own overrides locally
 .adk/

src/google/adk/memory/__init__.py

@@ -35,3 +35,20 @@
       ' VertexAiRagMemoryService please install it. If not, you can ignore this'
       ' warning.'
   )
+
+try:
+  from .database_memory_service import DatabaseMemoryService
+  from .memory_search_backend import KeywordSearchBackend
+  from .memory_search_backend import MemorySearchBackend
+
+  __all__ += [
+      'DatabaseMemoryService',
+      'KeywordSearchBackend',
+      'MemorySearchBackend',
+  ]
+except ImportError:
+  logger.debug(
+      'SQLAlchemy or an async DB driver is not installed. If you want to use'
+      ' DatabaseMemoryService please install sqlalchemy and an async driver'
+      ' (e.g. aiosqlite for SQLite). If not, you can ignore this warning.'
+  )