feat: run HTTP tools/call handlers on a dedicated worker task

[abcsnowwolf]

• execute tools/call on a queue-fed worker task instead of the async_tcp task, so slow or blocking handlers no longer starve the TCP stack or trip the task watchdog
• deliver results via a deferred chunked response (RESPONSE_TRY_AGAIN); protocol methods, notifications, and transport errors stay inline
• bound pending calls with MCP_HTTP_JOB_QUEUE_DEPTH (default 4) and answer JSON-RPC -32000 "server busy" when the queue is full
• manage jobs with an intrusive refcount allocated via new (std::nothrow): allocation failure answers JSON-RPC -32603 / HTTP 500 echoing the request id, in both exception modes, instead of aborting under -fno-exceptions; degrade to inline execution if the worker task cannot be created
• reject HTTP/1.0 tools/call with 505 instead of trusting the fork-dependent close-delimited fallback behind __has_include
• extend the ESPAsyncWebServer mock with chunked-response support and add tests: slow handlers, queue overflow, client abort, teardown with in-flight jobs, OOM injection, HTTP/1.0 rejection
• document the HTTP execution model in the README
• bump version to 0.3.0 and point examples at ESP-MCP ^0.3.0