refactor: use MetadataFilteringCondition BaseModel in RetrievalService.external_retrieve

[gaurav0107]

Fixes part of #31497.

Summary

Follow the dict -> Pydantic BaseModel pattern established by #31514, #34080, and #34422: RetrievalService.external_retrieve now accepts metadata_filtering_conditions: MetadataFilteringCondition | None directly instead of dict[str, Any] | None.

The redundant internal MetadataFilteringCondition.model_validate(...) call is removed; validation now happens at the call site (HitTestingService.external_retrieve), following the "validate at the boundary" principle cited in the issue.

Unit tests are updated to pass typed instances where appropriate, and a regression test is added for the None path. Behavior is otherwise preserved. external_retrieval_model is left as dict[str, Any] because it is an opaque payload forwarded to external retrieval providers (no single BaseModel describes the union of provider-specific options).

Screenshots

Before	After
N/A (refactor, no UI change)	N/A (refactor, no UI change)

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

[github-actions]

Pyrefly Diff

base → PR

--- /tmp/pyrefly_base.txt	2026-05-07 06:31:55.825147961 +0000
+++ /tmp/pyrefly_pr.txt	2026-05-07 06:31:46.255042949 +0000
@@ -5875,15 +5875,15 @@
 ERROR Cannot index into `object` [bad-index]
    --> tests/unit_tests/services/hit_service.py:462:16
 ERROR Cannot index into `list[Unknown]` [bad-index]
-   --> tests/unit_tests/services/hit_service.py:497:20
+   --> tests/unit_tests/services/hit_service.py:503:20
 ERROR Cannot index into `object` [bad-index]
-   --> tests/unit_tests/services/hit_service.py:497:20
+   --> tests/unit_tests/services/hit_service.py:503:20
 ERROR Argument `dict[str, str] | list[Unknown] | object` is not assignable to parameter `obj` with type `Sized` in function `len` [bad-argument-type]
-   --> tests/unit_tests/services/hit_service.py:498:24
+   --> tests/unit_tests/services/hit_service.py:504:24
 ERROR Cannot index into `list[Unknown]` [bad-index]
-   --> tests/unit_tests/services/hit_service.py:531:20
+   --> tests/unit_tests/services/hit_service.py:539:20
 ERROR Cannot index into `object` [bad-index]
-   --> tests/unit_tests/services/hit_service.py:531:20
+   --> tests/unit_tests/services/hit_service.py:539:20
 ERROR Object of class `Marketplace` has no attribute `repo`
 ERROR Object of class `FunctionType` has no attribute `call_args` [missing-attribute]
   --> tests/unit_tests/services/plugin/test_oauth_service.py:50:34