feat: enable SearchV PREFILTER support in HQL

[aiSynergy37]

Summary

• enable parser grammar for SearchV<...>(..., k)::PREFILTER(...)
• wire prefilter generation into root SearchV analysis path (start-node and expression forms)
• align generated prefilter closure binding to val so generated predicates compile against &HVector
• guard graph-step ::SearchV from silently dropping prefilters by surfacing an analyzer error
• add parser/analyzer tests covering PREFILTER parsing, valid simple predicates, and invalid non-boolean traversal usage

Notes

• this implementation intentionally supports simple property predicates (for example: _::{category}::EQ("tech")) for prefilter closures

Validation

• unable to run cargo test in this environment because cargo is not installed

Closes #834

Greptile Summary

This PR enables SearchV<T>(..., k)::PREFILTER(...) in HQL by uncommenting the grammar rule, wiring build_search_vector_pre_filter into the root SearchV analysis path, aligning the generated closure binding to val, and blocking PREFILTER on graph-step SearchV with an E601 diagnostic. Support is intentionally scoped to simple property predicates (_::{field}::BoolOp(value)).

Important Files Changed

Filename	Overview
helix-db/src/grammar.pest	Uncomments the optional ("::" ~ pre_filter)? suffix on search_vector, enabling PREFILTER syntax in HQL.
helix-db/src/helixc/analyzer/methods/infer_expr_type.rs	Adds build_search_vector_pre_filter and validator helpers; test fixtures use [F64] with an [F32] schema and compound And/Or prefilter paths are untested.
helix-db/src/helixc/analyzer/methods/traversal_validation.rs	Wires build_search_vector_pre_filter into the root SearchV analysis path, replacing the previously dead let pre_filter = None.
helix-db/src/helixc/analyzer/methods/graph_step_validation.rs	Guards graph-step SearchV from silently dropping prefilters by emitting E601 when sv.pre_filter is set.
helix-db/src/helixc/generator/source_steps.rs	Changes closure binding from v to val so generated predicates compile against the val identifier emitted by the traversal renderer; doc comment is now stale.
helix-db/src/helixc/parser/expression_parse_methods.rs	Adds a parser-level test confirming that SearchV<T>(..., k)::PREFILTER(...) round-trips through HelixParser::parse_source without error.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["HQL: SearchV<T>(vec, k)::PREFILTER(expr)"] --> B{Root or graph-step?}
    B -->|Root StartNode| C["traversal_validation.rs\nbuild_search_vector_pre_filter()"]
    B -->|Graph step| D["graph_step_validation.rs\nEmit E601 error"]
    C --> E["infer_expr_type()\nparent_ty = Type::Vector(T)"]
    E --> F{stmt result}
    F -->|Traversal| G["BoExp::Expr(tr)"]
    F -->|BoExp| H["BoExp directly"]
    F -->|Other / None| I["Emit E306 / E601\nreturn None"]
    G --> J["is_supported_search_vector_prefilter_expr()"]
    H --> J
    J -->|Valid: PropertyFetch + BoolOp| K["Some(vec![pre_filter])"]
    J -->|Invalid: Exists / Empty / wrong steps| L["Emit E601\nreturn None"]
    K --> M["SearchVector { pre_filter: Some(...) }\nsource_steps.rs Display:\n|val: &HVector, txn: &RoTxn| ..."]
    L --> N["SearchVector { pre_filter: None }"]

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_{Reviews (1): Last reviewed commit: "fix(hql): silence unused txn in generate..." | Re-trigger Greptile}

[aiSynergy37]

@xav-db linking this here for visibility: this PR implements issue #834 (SearchV PREFILTER support) with parser + analyzer/codegen wiring and tests.

[greptile-apps]

txn will always be unused in valid prefilter closures

The closure parameter txn: &RoTxn is declared but is never referenced in the generated body for valid prefilter expressions. The validator in is_supported_search_vector_prefilter_traversal restricts accepted traversals to exactly two steps (PropertyFetch + BoolOp), and bool_ops.rs renders those as val.get_property(...).map_or(false, |v| ...) — a path that only touches val. Any traversal needing txn (i.e., the multi-step G::from_iter(&db, &txn, ...) path) is already rejected by the validator. This will emit an unused-variable warning on every compiled prefilter closure; use _txn to suppress it.

Suggested change

	.map(|f| format!("|val: &HVector, txn: &RoTxn| {f}"))
	.map(|f| format!("|val: &HVector, _txn: &RoTxn| {f}"))

[aiSynergy37]

Addressed the unused xn warning in generated prefilter closures by renaming it to _txn in codegen.\n\nFix commit: 6d56b81

[aiSynergy37]

Opened a separate CI-fix PR for the cli_tests.yml parser failure noise: #900