fix(resource): add overlay to ResourceTypeConfigSchema._VALID_STRATEGIES

Implemented test coverage and documentation updates to ensure overlay is properly recognized and communicated across the sandbox strategy validation surface.

- Verification: Confirmed that overlay already exists in the _VALID_STRATEGIES frozenset within src/cleveragents/resource/schema.py (introduced in a prior commit that closed issue #2827).
- Comprehensive audit: Audited strategy validation points to ensure consistency:
  - _VALID_STRATEGIES in schema.py
  - _IMPLEMENTED_STRATEGIES and _SUPPORTED_STRATEGIES in factory.py
  - SandboxStrategy enum definitions in both resource.py and resource_type.py
  All include overlay as an allowed strategy.
- Tests: Added a new BDD scenario titled: "Invalid sandbox_strategy error message lists overlay as allowed" to features/consolidated_resource.feature. This asserts that validation errors explicitly list overlay among the allowed values when an invalid strategy is used.
- Documentation: Updated docstring in src/cleveragents/infrastructure/sandbox/strategy_registry.py to reflect that the built-in SandboxFactory handles six strategies, including snapshot and overlay.
- Design considerations: The fix itself was already applied in a previous commit (dbf3b8d7) addressing #2827. Issue #3614, raised by the UAT bot, required additional test coverage and accurate documentation of the available strategies.
- Impact: No behavioral changes beyond ensuring test coverage and documentation are aligned with the existing implementation. The added test guarantees that error messages remain accurate and comprehensive as strategies evolve.
- Affected areas: resource/schema.py, factory.py, resource.py, resource_type.py, features/consolidated_resource.feature, strategy_registry.py.

ISSUES CLOSED: #3614

features/consolidated_resource.feature

@@ -895,3 +895,9 @@ Feature: Consolidated Resource
     Given a resource type YAML with sandbox_strategy "overlay"
     When I load the resource type via from_yaml
     Then the loaded schema sandbox_strategy should be "overlay"
+
+
+  Scenario: Invalid sandbox_strategy error message lists overlay as allowed
+    Given a resource type YAML with invalid sandbox_strategy
+    When I try to load the resource type via from_yaml
+    Then the schema loader should fail with "overlay"

src/cleveragents/infrastructure/sandbox/strategy_registry.py

@@ -81,8 +81,8 @@ class SandboxStrategyRegistry:
     :class:`SandboxStrategyProtocol` at registration time.
 
     Built-in strategies (``none``, ``git_worktree``, ``copy_on_write``,
-    ``transaction_rollback``) are not managed by this registry; they
-    are handled by :class:`SandboxFactory`.
+    ``transaction_rollback``, ``snapshot``, ``overlay``) are not managed
+    by this registry; they are handled by :class:`SandboxFactory`.
 
     Usage::