[mauripunzueta]

Closes #2007.

What

On MongoDB, the check for whether a :missing parameter is answered from missing_params ran again at every stage of a search. Each run took the tenant registry read lock and built a new SearchParameterExtractor. This PR classifies the parameters once per query and passes the result through planning and execution.

• New MissingSupport (in search_impl.rs), built by MongoBackend::classify_missing(tenant, resource_type, query). It takes one registry read lock and builds one extractor per query. It stores the (name, param_type) pairs that missing_params answers. The old check depended only on those two fields (plus the index-ready flag), so the stored set gives the same answers it did.
• supports_indexed_missing and is_missing_resource_parameter are gone. Their callers use MissingSupport::supports and MissingSupport::is_resource_parameter instead.
• The value is passed through resource_filter_plan, plan_or_matching_ids, matching_resource_ids (and its seeded, complement-only and candidate-retention helpers), build_resource_filter, resource_order_page, sparse_page_id_set, search_param_sorted/resource_level_ids and count_standard_with_facts.
• search_inner computes it once. Its total count reuses that value and the same database handle, where it used to call get_database() again.
• retain_matching_candidates now receives the caller's db and no longer calls self.get_database() for each metadata-backed missing parameter.
• The planner and the executor now read the same classification. Before, a registry reload in the middle of a query could make them disagree.
• The _contained=both top-level filter is classified against the contained type, which is the type it already used.
• The storage.rs conditional-match callers only pass _id / _lastUpdated parameters, so they use MissingSupport::none(). Behaviour there is unchanged.

Acceptance criteria

• One classification per parameter per query
• No get_database() inside per-parameter loops
• Existing missing-index and resource-page tests still pass (see Testing)

Testing

Locally, against a single-node MongoDB 8.0.13 replica set, with HFS_TEST_REQUIRE_MONGODB=1 so that no Mongo-backed test could skip:

• cargo test -p helios-persistence --features mongodb --lib backends::mongodb → 486 passed, 0 failed. This includes the planner's execution_tests and resource_page_tests.
• cargo test -p helios-persistence --features mongodb --test mongodb_tests → 506 passed, 0 failed, 2 ignored (the #[ignore]d benchmarks).
• cargo clippy -p helios-persistence --features mongodb --tests: no new warnings. The only warnings in the files touched are existing collapsible_if lints from the newer local toolchain, none of them on lines this PR changes.

No new test is added. The change is a refactor that should not change behaviour, and the existing missing-index tests cover each branch that now reads MissingSupport.

[codecov]

Codecov Report

❌ Patch coverage is 90.10417% with 19 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...es/persistence/src/backends/mongodb/search_impl.rs	93.52%	11 Missing ⚠️
crates/persistence/src/backends/mongodb/storage.rs	20.00%	8 Missing ⚠️

📢 Thoughts on this report? Let us know!