feat: WithSchemaValidation — JSON Schema arg validation

[inhuman]

Summary

• WithSchemaValidation() Option — enables client-side validation of CallTool args against each tool's InputSchema before the call is dispatched
• Validation uses xeipuuv/gojsonschema (already an indirect dep via dockertest, now promoted to direct — no new download)
• Empty/nil args are treated as {}, so required-field violations are caught even when no args are passed
• Disabled by default; tools without an InputSchema are skipped
• Violations returned as *ErrInvalidArgs with SchemaErrors []string field populated

Test plan

• 8 unit tests in validate_test.go for validateSchema (empty schema, valid args, missing required, wrong type, constraints, empty args)
• 6 integration tests in caller_test.go (valid pass-through, missing required, wrong type, empty args + required, disabled by default, no schema skips)
• api_surface_test.go — WithSchemaValidation pinned
• go test -race ./... — all pass
• golangci-lint run ./... — 0 issues
• CI passes