Improve gojq usage: ValueError handling, WithVariables, filter cache, inferSchema decoupling, any alias

[Copilot]

Addresses all five recommendations from the gojq module review (#7670): better runtime error diagnostics, injection-safe variable binding, compile-time filter caching, decoupled schema logic, and idiomatic Go 1.25 types.

Error handling (runJqCode)

• Added gojq.ValueError check between HaltError and the generic fallback — intentional error() builtin calls get a clean message; all unexported type errors (null access, wrong-type builtins) get a "check filter handles null/missing fields" hint
• Note: gojq.TypeError from the issue report does not exist in the public API; gojq.ValueError is the correct interface

Variable injection (CompileToolResponseFilterWithVars)

New function using gojq.WithVariables for per-call context without string interpolation:

code, _ := CompileToolResponseFilterWithVars(
    `if $serverID == "github" then .items[:20] else . end`,
    []string{"$serverID"},
)
iter := code.RunWithContext(ctx, data, serverID)

validateToolResponseFiltersWithVars added in config/validation.go to mirror the same compile options for fail-fast startup validation.

Filter compilation cache (filterCodeCache sync.Map)

• CompileToolResponseFilter checks the cache before parsing; stores on miss
• Cache is unbounded but bounded in practice by the number of distinct filter strings in config
• Uses checked type assertion (cached.(*gojq.Code)) to avoid potential panics on unexpected cache contents

inferSchema decoupled from gojq

• Moved to internal/middleware/schema.go — imports only encoding/json and reflect, no gojq dependency
• Enables direct calls and independent testing without the gojq import

interface{} → any

All internal and public signatures in jqschema.go updated to any (idiomatic Go 1.18+, project targets Go 1.25).

[Copilot]

Pull request overview

This PR refines the gateway’s jq (gojq) integration in internal/middleware by improving runtime error diagnostics, adding an injection-safe compilation path for variable-bound filters, caching compiled filter bytecode, and separating schema inference into a gojq-independent helper. It also updates middleware-facing APIs to use Go’s any alias consistently.

Changes:

• Added gojq.ValueError handling in runJqCode to distinguish intentional error(...) failures from runtime type errors and include a null-safety hint for the latter.
• Introduced CompileToolResponseFilterWithVars (uses gojq.WithVariables) and added a sync.Map cache for compiled non-parameterized filters.
• Moved inferSchema into a new internal/middleware/schema.go to decouple schema inference from gojq imports, and updated interface{} → any across the middleware surface.

Show a summary per file

File	Description
internal/middleware/schema.go	New gojq-independent inferSchema implementation for schema inference.
internal/middleware/jqschema.go	Adds filter compilation cache, ValueError vs runtime-type error handling, WithVariables compilation helper, and any type updates.
internal/middleware/jqschema_coverage_test.go	Adds coverage for ValueError/type-error paths, WithVars compilation, and cache hit behavior.
internal/config/validation.go	Adds validateToolResponseFiltersWithVars helper to mirror WithVariables compile options for startup validation.
internal/config/validation_test.go	Tests the new validateToolResponseFiltersWithVars validation helper.

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• Files reviewed: 5/5 changed files
• Comments generated: 0