test: raise memory thresholds in memoryUsage namespace test

[Tony133]

Proposal:

The memoryUsage name space test registers the plugin with maxRssBytes: 100000000 (100 MB) and a pressureHandler that asserts it is never called. That threshold is too close to the actual footprint of a modern Node.js process: on Linux with Node 22 the RSS at that point of the test file is already ~95 MB, and on macOS with Node 26 (added to the matrix by fastify/workflows v7.0.0) with coverage enabled it exceeds 100 MB.

When that happens the plugin invokes the pressureHandler, t.assert.ok(false) throws inside the hook, Fastify turns it into a 500 and the test fails with an opaque message:

    AssertionError [ERR_ASSERTION]: 500 == 200
        at test/index.test.js:283:18

This is what blocks #296.

The test only verifies that the fastify.memoryUsage() namespace works and that no pressure is detected; it does not exercise the thresholds. This PR:

• raises maxHeapUsedBytes / maxRssBytes to 1e9 and maxEventLoopUtilization to 0.99 so they cannot be hit by the runtime footprint
• adds a message to the t.assert.ok(false) reporting which metric triggered the handler and with which value
• passes the response body as the message of the status code assertion, so that message actually shows up in the test output instead of being swallowed by the 500 response

Note:

• No changes to index.js, behaviour under test is unchanged.

[jean-michelet]

Sounds good, let's wait for Matteo review imo.

[mcollina]

lgtm