feat(search): scope memory search by workspace

[yordis]

• Memories already support being owned by either a workspace or a project, but the search surface (MCP search_memories, REST GET /api/memory/search, and the underlying HybridSearch / SearchWithMetadataEmbedding) only exposed project-level scoping, leaving no way to query "everything in this workspace" in a single call.
• Agents and tools that operate at the workspace level (cross-project context, team-wide notes, broader recall) had to either issue N project queries and merge, or fall back to global search — both lose precision and waste tokens.
• Scope is intentionally limited to direct workspace + its directly-owned projects so this change is small and reviewable; sub-workspace recursion is a follow-up once the simple shape proves itself.
• projectId and workspaceId are kept mutually exclusive so the API stays unambiguous and easy to evolve — adding union/intersect semantics later is reversible; reserving the namespace now is not.

[Aaronontheweb]

I patched the review findings on branch petabridge/memorizer:aaron/pr-196-workspace-search-fixes at commit 751d9bd.

Fixes included:

• REST GET /api/memory/search and GET /api/memory/search/metadata now accept and forward workspaceId scope, with projectId / workspaceId mutual-exclusion validation.
• MCP SearchMemories now rejects malformed non-empty projectId / workspaceId values instead of silently widening to global search.
• Added regression tests for MCP workspace forwarding, malformed workspace IDs, REST workspace forwarding on both search endpoints, and REST mutual-exclusion validation.

Verification:

• dotnet test /home/aaronontheweb/repositories/petabridge/memorizer-v1/src/Memorizer.UnitTests/Memorizer.UnitTests.csproj --no-restore from outside the repo root: passed, 171 tests.
• dotnet test /home/aaronontheweb/repositories/petabridge/memorizer-v1/src/Memorizer.IntegrationTests/Memorizer.IntegrationTests.csproj --no-restore --filter FullyQualifiedName~McpToolsProjectScopingTests from outside the repo root: passed, 10 tests.

I could not push directly to the PR head fork TrogonAi/memorizer; GitHub returned 403 for my account, so the fixes are available on the Petabridge branch above.

[yordis]

@Aaronontheweb I put your changes in it