fix: accept null tool arguments and bound HTTP resource use

Review follow-ups on the SDK migration.

An "arguments": null is what clients send for parameterless tools like get_me,
and what mcp-go accepted by returning a nil map. The new adapter rejected it
with InvalidParams, which broke those calls outright.

The /mcp endpoint took unlimited request bodies and never expired idle
sessions, so a peer that goes away without DELETE kept its session for the
process lifetime. Both are reachable before any token check, so neither can
stay unbounded; the body cap sits above the SDK default to leave room for the
base64 content create_or_update_file accepts.

Required() smuggled a bool through the property schema map and deleted it
again, colliding with the JSON Schema keyword of the same name. It now sets a
field on Property, so an object property can carry its own required list.

The tool contract fixture cost a manual regeneration step and four
hand-maintained counts on every tool change, and a snapshot freezes defects
rather than reporting them. Property assertions cover the same surface and
reject a duplicate tool name, a readOnlyHint that disagrees with the register
call, and a default that contradicts its own type or enum.

Co-Authored-By: Claude (Opus 5) <noreply@anthropic.com>

Makefile

@@ -6,6 +6,8 @@ LDFLAGS := -X "main.Version=$(VERSION)"
 GOLANGCI_LINT_PACKAGE ?= github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.2 # renovate: datasource=go
 GOVULNCHECK_PACKAGE ?= golang.org/x/vuln/cmd/govulncheck@v1.3.0 # renovate: datasource=go
 
+GOTEST_FLAGS ?= -race -timeout 20m
+
 .PHONY: help
 help: ## print this help message
 	@echo "Usage: make [target]"
@@ -40,7 +42,7 @@ build: ## build the application
 
 .PHONY: test
 test: ## run Go tests
-	$(GO) test ./...
+	$(GO) test $(GOTEST_FLAGS) ./...
 
 .PHONY: air
 air: ## install air for hot reload