refactor: jq middleware improvements from gojq module review

[lpcox]

Summary

Implements the quick-win improvements identified in the gojq module review (#2985).

Changes

1. Remove duplicate test helper

Deleted integrationPayloadMetadataToMap from jqschema_integration_test.go — it was byte-for-byte identical to payloadMetadataToMap in jqschema_test.go. Both files are in package middleware, so the shared helper is directly usable.

2. Document custom walk vs built-in

Added a comment block explaining why jqSchemaFilter defines its own walk function instead of using gojq's built-in walk(f):

• Built-in walk(f) applies f post-recursion but preserves structure
• Our custom walk replaces leaf values with type names AND collapses arrays to first element
• These behaviors are incompatible with standard walk(f) semantics

3. Document single-result iterator contract

Added comment explaining why iter.Next() is called only once — the walk(.) filter produces exactly one output value.

4. UTF-8 safety comment on preview truncation

Documented why byte-level slicing at PayloadPreviewSize is safe: json.Marshal escapes non-ASCII runes as \uXXXX, so every byte boundary is a valid UTF-8 boundary.

5. Skip redundant JSON unmarshal

Added a type-switch in WrapToolHandler that skips the json.Unmarshal step when data is already map[string]interface{} or []interface{}. This avoids a needless marshal→unmarshal round-trip on hot paths.

Closes #2985

[Copilot]

Pull request overview

This PR applies “quick-win” improvements from the gojq module review (#2985) to the jq schema middleware, focusing on documentation clarity and avoiding unnecessary JSON processing on hot paths.

Changes:

• Removed a duplicate test helper in jqschema_integration_test.go in favor of the shared helper in jqschema_test.go.
• Added explanatory comments around the custom jq walk implementation and the single-result iterator contract.
• Added a type-switch optimization in WrapToolHandler to avoid redundant marshal → unmarshal when the payload is already map[string]interface{} / []interface{}.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
internal/middleware/jqschema.go	Adds documentation clarifications and an optimization to skip redundant unmarshaling before running gojq.
internal/middleware/jqschema_integration_test.go	Removes duplicated helper and reuses the existing shared test helper.

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[Copilot]

The UTF-8 safety rationale here is incorrect: encoding/json.Marshal emits UTF-8 for non-ASCII runes (it does not generally escape them as \uXXXX). Byte-slicing payloadJSON at an arbitrary boundary can split a multi-byte UTF-8 sequence, so the preview string may be invalid UTF-8. Either adjust the truncation to a valid rune boundary (or validate/repair), or update the comment to accurately describe the behavior and its implications for downstream JSON encoding/logging.