Negotiate modern and legacy protocol eras

[pushpak1300]

Currently the client always starts with the legacy initialize handshake. That locks it to the pre-2026-07-28 protocol flow and prevents it from talking to modern servers that expect discovery and per-request protocol metadata.

Existing calls do not need to choose a protocol version:

$client = Mcp::client('everything');

$client->tools();

$client->protocolVersion();
$client->discoverResult();
$client->initializeResult();

The client probes the latest protocol first, then falls back to the legacy handshake only when the response indicates that the endpoint does not support the modern flow. If the server reports its supported versions, the client selects the newest mutual version instead of guessing.

A version can also be pinned when the server is already known:

Mcp::client('everything')
    ->withProtocolVersion(ProtocolVersion::V2025_11_25)
    ->tools();

The HTTP transport keeps session handling in the legacy flow and sends protocol-version headers in the modern flow. Method and name header mirroring is intentionally left for a follow-up so it can be modeled at the method boundary instead of in the transport.

Warning

Custom client transports no longer implement setProtocolVersion(string). Protocol-aware transports should implement UsesProtocol::useProtocol(ProtocolVersion), while protocol-agnostic transports need no protocol method. ProtocolVersion::supported() is replaced by serverSupported(), with clientSupported() and initializeSupported() exposing the other support lists.

Tests cover negotiation, fallback behavior, version pinning, reconnects, protocol metadata, HTTP headers, and legacy sessions.