[Server] Allow pre-built instance handlers in element registration (#375)

* Allow pre-built instance handlers in element registration

`HandlerResolver::resolve()` rejected `[$instance, 'methodName']` array
handlers even though the `Handler` type alias and the runtime invoker
already support them, so `Server::builder()->addTool(handler: [$obj, 'bar'])`
threw "Invalid array handler format" at build time.

Relax the array-shape guard to accept an object in slot 0 and normalize it
to its class name for reflection (the same idiom getHandlerDescription()
already uses). This unblocks dependency-injected handler objects that the
container-less `new $className()` fallback cannot build.

Fixes #369

* Use 0.6.1 changelog heading and test instance handler end-to-end

This is a bugfix with no new public API or BC break, so the changelog
entry belongs under 0.6.1 rather than a new 0.7.0 section.

Add an end-to-end test that registers a pre-built instance handler with a
constructor dependency the container-less new $className() fallback cannot
satisfy, then drives a real tools/call through CallToolHandler and asserts
the injected instance is the one invoked.

* Address review: reject Closure array handlers, fix abstract-method message

- HandlerResolver: extract the array-handler guard into a readable
  $hasValidTarget check that accepts strings and objects but excludes
  \Closure, so [$closure, 'method'] falls through to the format error
  instead of the misleading "method not found in class Closure".
- Fix the abstract-method rejection message ("must be abstract" ->
  "must not be abstract"), a pre-existing typo in the same branch.
- Tighten the CHANGELOG entry wording.
- Cover Closure rejection with a regression test.

* Move instance-handler entry under 0.7.0 changelog heading

Per review: registering pre-built instance handlers is a feature, not a
patch, so it belongs in a minor release heading.

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 All notable changes to `mcp/sdk` will be documented in this file.
 
+0.7.0
+-----
+
+* Allow `[$instance, 'methodName']` as an element handler in `Builder::addTool()`, `addResource()`, `addResourceTemplate()`, and `addPrompt()`. Unblocks handlers with constructor dependencies that the container-less `new $className()` fallback cannot build.
+
 0.6.0
 -----