fix(templates): apply delay from go_template_json dynamic responses

[gwleclerc]

Fixes #305.

Problem

A delay written as duration strings in a go_template_json dynamic response was silently ignored — the response returned immediately — while the same mock worked under lua:

dynamic_response:
  engine: go_template_json
  script: >
    { "body": {...}, "delay": { "min": "0", "max": "10ms" } }

Root cause

Delay.UnmarshalJSON only accepted numeric nanoseconds, not duration strings like "10ms". The Lua engine pre-parses durations (time.ParseDuration) and go_template_yaml happens to accept the string form, but go_template_json feeds the raw JSON straight to Delay.UnmarshalJSON, which errored on the string. goTemplateJsonEngine.Execute logs that error but doesn't return it, so the delay was dropped as {0, 0} and the mock answered with no delay.

Fix

Parse each delay bound as either a duration string (time.ParseDuration) or a number of nanoseconds, matching the Lua/yaml engines. Marshalling is unchanged (still numeric nanoseconds), so the on-disk mock format is unaffected.

Verified

• New engine tests: object form {min,max} and scalar shorthand "5ms" parse correctly.
• Golden serialization + JSON-schema tests unchanged; full server suite green.
• Runtime: a go_template_json mock with delay {min:300ms, max:300ms} now responds in ~0.30s (was ~0s).

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