Hold every MCP tool's arguments to its exact member names

[kikashy]

Goal

Apply the exact member-name check experimental_test_graphs already uses to the other MCP tools that decode an arguments object.

Why

Every tool advertises additionalProperties: false, which means the exact spelling. encoding/json matches struct field names case-insensitively, so DisallowUnknownFields alone is not enough: a differently-cased member binds to the real field and the call succeeds.

internal/mcp/tools.go has an exactMembers helper for this, added with experimental_test_graphs, but it is used by that tool only. The others decode with DisallowUnknownFields alone.

Demonstrated against a built server, same argument shape, two different answers:

get_pack                  {"PACK_ID":"intake"}   -> isError=false, returns the pack
experimental_test_graphs  {"GRAPH_ID":"x"}       -> isError=true,  "unknown member"

The advertised schema is identical in kind for both. One of them enforces it.

Scope

• Change internal/mcp/tools.go and internal/mcp/server_test.go only.
• Call exactMembers before decoding in each tool that takes an arguments object and does not already: get_pack, experimental_evaluate, and experimental_test_packs.
• Keep each tool's existing error text shape; exactMembers already produces a message naming the offending member and the accepted ones.
• Do not change any tool's advertised schema, argument semantics, or success behaviour.

Acceptance criteria

• For each affected tool, a differently-cased member name (PACK_ID, Pack_Id, packId where the schema says pack_id) is refused.
• Refusals are MCP tool errors inside successful JSON-RPC responses, not top-level protocol errors.
• Correctly spelled arguments still succeed unchanged, and omitted-optional behaviour is untouched.
• Table-driven tests using the existing stdio harness, with t.Run per case.
• env GO111MODULE=on go test ./internal/mcp and go test ./... pass.
• Every commit includes a DCO sign-off created with git commit -s.

Contributor learning

MCP argument contracts, why a schema keyword needs an enforcing check behind it, and one concrete way Go's JSON decoding is more permissive than a schema.