[Upstream Update] agentgateway v1.3.1 -> v1.4.0

[clubanderson]

Upstream Dependency Update

Field	Value
Dependency	agentgateway
Current	v1.3.1
Latest	v1.4.0
Release	https://github.com/agentgateway/agentgateway/releases/tag/v1.4.0

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Nightly scan from llm-d-infra

[github-actions]

🤖 Auto-Fix Instructions

Copilot, please update the version pin for the dependency described in this issue.

Steps

1. Read docs/upstream-versions.md to find the dependency row, its File Location, and Pin Type
2. Parse the issue title to get the dependency name, old version, and new version
3. If the current pin is auto, latest, stable, or unpinned (floating pin type), comment that no change is needed and stop
4. Update the version in the source file mentioned in the File Location column
5. Update the Current Pin value in docs/upstream-versions.md
6. Validate modified shell scripts with bash -n

PR Format

• Title: ⬆️ Bump {dependency} from {old_version} to {new_version}
• Body: Reference this issue with Closes #1680
• Keep changes minimal — only update the version pins
• Sign commits with DCO (git commit -s)