Make invoke_tool self-correcting on argument mismatch

[rockfordlhotka]

Problem

Claude Desktop (via this aggregator) repeatedly failed calling the downstream csla-mcp search/fetch tools with missing required parameter 'message' / 'fileName'. The downstream server, its API key, and its embedding model were all healthy — a direct tools/call with the correct keys works.

Root cause: list_services returns only each tool's name + description, not its inputSchema (ServiceIndex.ToolSummary). When a consumer invokes off list_services alone, it guesses parameter names from the description prose (e.g. csla-mcp's message parameter is described with the word "query," steering the guess to query/name). The downstream SDK then sanitizes the binding error to a generic "An error occurred invoking '<tool>'", so the model has no way to recover without separately calling get_service_details.

Fix

ToolProxyHandler.InvokeAsync is now self-correcting. When a downstream call returns an error, it compares the supplied arguments against the tool's cached input schema and — only when there's a real mismatch (missing required or unknown keys) — appends a corrective block to the error result naming the specific bad keys and the full schema. The model can then retry correctly without a separate get_service_details round-trip.

• Genuine tool-side errors on schema-valid arguments are left untouched (no noise).
• Schema lookup reuses the ToolIndex cache and is fully guarded, so it can never break the proxy.

Example of the enriched error a caller now receives:

Argument mismatch for tool 'search' on 'csla-mcp'. Missing required parameter(s): [message]. Unrecognized argument key(s): [query]. You sent: [query]. Re-invoke with arguments matching this input schema: {…"required":["message"]…}

Verification

• dotnet build clean (0 warnings/errors); SDK types matched.
• 88/89 tests pass; the one failure is the pre-existing flaky JsonFilePersistence parallel-IO test, which passes in isolation and is unrelated to this change.

Version bumped 0.3.0 → 0.3.1.

Follow-up (not in this PR)

Optionally add compact param hints (required parameter names) to list_services itself, so the wrong guess is prevented at the source as well.

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