docs: add build & test guidance for coding agents (AGENTS.md)

[milamberspace]

What

Adds a short "Building and testing (for coding agents)" section to
AGENTS.md so any AI coding agent has JMeter's build context up front —
independent of any agent framework.

Follows the suggestion on the dev@ DISCUSS thread
to give agents JMeter-specific build knowledge (in particular, how to run
tests under a specific JDK version).

Contents

• Core gradlew tasks: build, test, :src:core:test, check,
  runGui, createDist, style.
• JDK selection for build and tests via -PjdkBuildVersion /
  -PjdkTestVersion / -PjdkTestVendor, with a copy-paste example
  (run the suite on JDK 21 / Corretto) — the exact flag agents were
  missing.
• Dependency-checksum refresh (-PupdateExpectedJars check), coverage,
  and RAT tasks.
• Pointer to the CI matrix and the Error Prone job.

Pure documentation; no code or build changes. Points to the existing
gradle.md and README.md "Test builds" section rather than duplicating
them.

🤖 Generated with Claude Code

[vlsi]

I would avoid writing "common sense" in agents.md

[vlsi]

Suggested change

	## Building and testing (for coding agents)
	## Building and testing

[vlsi]

Suggested change

	JMeter builds with Gradle. Use the wrapper `./gradlew` (or `gw` from
	[gdub](https://github.com/dougborg/gdub)). The full command list is in
	[`gradle.md`](gradle.md), and toolchain details are in the
	[Test builds](README.md#test-builds) section of `README.md`. The essentials:
	- **Build everything (incl. tests + static checks):** `./gradlew build`
	- **Unit tests only:** `./gradlew test` — single module, e.g. `./gradlew :src:core:test`
	- **All checks (tests, checkstyle, spotless, …):** `./gradlew check`
	- **Run the GUI from source:** `./gradlew runGui`
	- **Assemble a runnable dist:** `./gradlew createDist && ./bin/jmeter`
	- **Format before committing:** `./gradlew style` (spotlessApply + checkstyleAll); check-only: `./gradlew spotlessCheck checkstyleAll`
	- **Skip tests:** append `-x test`
	JMeter builds with Gradle. Use `./gradlew --quiet ...` to reduce output verbosity.
	After modifying Java code, before reporting the change as done (handing control back to the user), run `./gradlew --quiet classes style` and fix any reported issues.

Agents already know Gradle commands. There's no point in duplicating them like ./gradlew build.

[vlsi]

Suggested change

	JMeter uses Gradle [toolchains](https://docs.gradle.org/current/userguide/toolchains.html),
	so JDKs are found locally or auto-provisioned. The default build JDK is 17
	(artifacts target Java 8). To build or test under a specific JDK — the way CI
	runs its matrix — pass build parameters:
	- `-PjdkBuildVersion=<n>` — JDK to build with (e.g. `21`)
	- `-PjdkTestVersion=<n>` — JDK to run **tests** with; `0` means "use the current Java"
	- `-PjdkTestVendor=<vendor>` / `-PjdkTestImplementation=<impl>` — pin the vendor / VM implementation
	Example — run the test suite on JDK 21 (Corretto):
	```sh
	./gradlew test -PjdkTestVersion=21 -PjdkTestVendor=corretto
	```
	List every available build parameter with `./gradlew parameters`.
	JMeter uses Gradle toolchains; JDKs are found locally or auto-provisioned. The code targets JDK 17. To test under a specific JDK pass build parameters:
	- `-PjdkTestVersion=<n>` — JDK to run **tests** with; `0` means "use the current Java"
	- `-PjdkTestVendor=<vendor>` / `-PjdkTestImplementation=<impl>` — pin the vendor / VM implementation (jdkTestVersion is enough for most cases)
	List every available build parameter with `./gradlew parameters`.

[vlsi]

Suggested change

	CI mirrors these across a JDK × vendor × OS × timezone × locale matrix (see
	[`.github/workflows/main.yml`](.github/workflows/main.yml)); the Error Prone
	static-analysis job runs on JDK 21.