Fix dart3_native regression

[iapicca]

Description

This PR partially rolls back changes introduced in #10493.

Motivation

Inter-process port sharing is not supported (see dart-lang/sdk#62545).
As a result, the horizontal “hybrid” scaling approach introduced in the referenced PR does not function correctly.
This causes both performance and image size regressions.

Changes

• Reimplemented the multi-stage Dockerfile using scratch instead of busybox
• Updated the Dart version
• Simplified request handling
• General cleanup (e.g., removing unnecessary Futures and async usage)

[p8]

@iapicca Unrelated to this PR, but I'm seeing the following warning on master:

Framework dart3 does not define a default test in benchmark_config.json

[iapicca]

@iapicca Unrelated to this PR, but I'm seeing the following warning on master:

Framework dart3 does not define a default test in benchmark_config.json

Hi @p8
I'm happy to include the fix in the current PR, but it's not clear to me what causes the warning.
The default seems to be already defined in dart3 benchmark_config.json , am I missing something?

Thank you

[p8]

@iapicca I think you need to delete these:

FrameworkBenchmarks/frameworks/Dart/dart3/benchmark_config.json

Lines 17 to 18 in a650250

	},
	{

[iapicca]

@p8 done, thank you

[p8]

Nice!