[Server] Add ClientGateway::supportsSampling()

[chr-hertel]

Sampling was the one client capability a tool could not check before asking.

supportsRoots() and supportsElicitation() both read the capabilities the client advertised during the handshake. Sampling is stored in that same session data — ClientCapabilities::jsonSerialize() emits it alongside roots and elicitation — but had no accessor, so a tool had to issue the request and catch the refusal:

// before
try {
    $result = $context->getClientGateway()->sample($prompt);
} catch (ClientException) {
    // client cannot sample
}

// after
if ($context->getClientGateway()->supportsSampling()) {
    $result = $context->getClientGateway()->sample($prompt);
}

Four lines mirroring supportsElicitation(), plus unit tests matching the existing supportsRoots pair.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a server-side capability accessor for client sampling support, enabling tools to check the client’s advertised sampling capability before attempting sampling/createMessage requests (consistent with existing supportsRoots() / supportsElicitation() patterns).

Changes:

• Add ClientGateway::supportsSampling() which reads the advertised client_capabilities session data.
• Add unit tests covering supportsSampling() returning true/false based on advertised capabilities.
• Document the new accessor in the changelog for the next release.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/Server/ClientGateway.php	Adds supportsSampling() to check for the sampling capability in session-stored client capabilities.
tests/Unit/Server/ClientGatewayTest.php	Adds tests verifying supportsSampling() behavior when sampling is present/absent.
CHANGELOG.md	Notes the addition of ClientGateway::supportsSampling() and its intended usage.

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