dredd-transactions: OpenAPI 3.1 schema edge cases

[dalberola]

OpenAPI-3.1-specific gaps in sample generation / parsing (agent-reported; [needs repro]). Relevant to this fork's OpenAPI-3 focus.

• Multi-type schemas sample only the first non-null type. packages/dredd-transactions/compile/openapi31.js: schemaTypes(resolvedSchema).filter((t) => t !== 'null')[0]. For type: ["object","string"] only object is ever exercised.
• Dead param.type switch. packages/dredd-transactions/compile/compileURI/validateParams.js switches on param.type, but compileParams.js never populates a type field — the type-validation branch is unreachable.
• allOf of non-objects yields undefined instead of a sensible sample (openapi31.js allOf handler).
• YAML parse errors swallowed. packages/dredd-transactions/parse/index.js: yaml.load failure is caught and discarded, so users get a generic "unrecognized format" instead of the YAML error.

Tasks

• Decide sampling behavior for multi-type schemas (and document it).
• Remove the dead param.type switch, or populate type upstream so validation runs.
• Handle allOf with primitive subschemas.
• Surface YAML parse errors as annotations when the doc is neither valid 3.0 nor 3.1.
• Fixtures for each 3.1 construct above.