Fix inbound command payload truncation at 1000 wide chars in connector DLLs

[biohazardxxx]

Summary

Inbound command payloads (client → EA) were silently truncated at 1000 wide characters by convertSystemString in both connector DLLs, even though the experts allocate a 5000-character payload buffer (StringInit(payload, 5000, 0)). Any command whose JSON payload exceeded 1000 characters arrived cut off mid-JSON, failed parsing in the EA and surfaced as Failed to get payload — an error that looks like a transport bug. The helper also never wrote a null terminator, so a shorter error message written into a reused _error buffer could leak the tail of a previous, longer one.

This PR removes the cap entirely:

• convertSystemString(dest, src, capacity) now receives the destination capacity, bounds the copy and always null-terminates.
• getPayload keeps its signature (old EAs keep working) but honors the expert's real 5000-character buffer instead of 1000.
• New size-aware export in both DLLs:

  // returns the full payload length in wchar_t (excluding null), or -1 on error;
  // copies at most capacity-1 chars and always null-terminates
  int getPayload2(int expertHandle, wchar_t* res, int capacity, wchar_t* err);

• GetJsonPayload in both experts (MQL4 + MQL5) fetches via getPayload2 with the default 5000-character buffer and, when the reported size is larger, re-allocates to the exact size and fetches again — one connector round-trip in the common case, two for oversized payloads.

Implementation notes

• Re-reading the payload is safe: MtExpert::GetCommandPayload() reads current_task_ non-destructively; the task is only consumed on SendResponse.
• MtCommand/WebSocket transport already handle arbitrary payload sizes — the cap was exclusively in this one copy helper.
• Applies identically to MT5Connector and MTConnector; recompiled MtApi5.ex5 and MtApi.ex4 are included.
• Old EA + new DLL remains compatible (old getPayload path, now capped at 4999 instead of 1000). New EA requires the new DLL (imports getPayload2); DLL and expert ship together in the installer.

Testing

• MT5Connector (x64) and MTConnector (Win32) build clean in Release; MtApi5.mq5 and MtApi.mq4 compile with Result: 0 errors, 0 warnings.
• Runtime-verified against a live MT5 terminal (build 5830) with the new DLL deployed and the recompiled EA attached:
  • Print with a 6015-character message (payload above even the 5000 default buffer, exercising the re-allocate path): returns true, EA parses and executes the command.
  • Control on the same terminal, same DLL, but an expert running the old .ex5: the same 6015-character command fails with Failed to get payload — reproducing the original defect and confirming the fix is what makes the difference.
  • Short-payload sanity command on the new stack: unchanged behavior.
• MQL4 expert change is source-identical to the MQL5 one and compiles clean, but was not runtime-tested against an MT4 terminal.

Removes the payload-size caveat documented in #307 (struct-array commands no longer need client-side chunking once both land).

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