refactor: Introduce AbstractClientTransport and simplify client transport lifecycle (#517) (#516)

## refactor: Introduce AbstractClientTransport and simplify client
transport lifecycle

This PR unifies state management logic across client transports, except
for the websocket transport, as it needs more refactoring first and is
not the primary transport.

- Introduce `AbstractClientTransport` as base class for other client
transports.
- Migrate to `ClientTransportState` states with state machine,
streamline state management.
- Ensure proper cleanup and error handling consistency across transport
implementations.
- Add unit tests to validate `AbstractClientTransport` behavior and
state transitions.
- Update related documentation and API contract.
- Make sure `onClose` callback is called at most once
- fix
[StreamableHttpClientTransportTest.kt‎](https://github.com/modelcontextprotocol/kotlin-sdk/pull/516/changes#diff-b9fe8891d08a492448a7ed2dc782151c6a915ae1403a4d70c2034c6062c84cb5)
to throw `McpException` on connect when received invalid Json

## Motivation and Context

Unify lifecycle handling and eliminate duplication in client Transport
implementations.


## How Has This Been Tested?
Unit test, regression tests

## Breaking Changes

- `onClose` callback is now called only once
- send(...) will throw McpException with message "Transport is not
ready" when `ClientTransportState != Operational`

## Types of changes

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] Refactoring
- [ ] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
See #517

conformance-test/src/test/kotlin/io/modelcontextprotocol/kotlin/sdk/conformance/ConformanceServer.kt

@@ -417,6 +417,6 @@ private class HttpServerTransport(private val sessionId: String) : AbstractTrans
         logger.debug { "Closing transport for session $sessionId" }
         messageQueue.close()
         pendingResponses.clear()
-        _onClose.invoke()
+        invokeOnCloseCallback()
     }
 }