[cbrady]

Adds sign_context, which a customer's backend calls to sign the company and user a browser SDK sends with a publishable key, for environments that require context signatures. It returns the X-Schematic-Context-Sig value the JS SDK's getContextSignature expects. A signature lasts 10 minutes by default; lifetime_seconds accepts up to the API's one-hour maximum, and issued_at sets the start time. It matches the Go and Node helpers (SchematicHQ/schematic-go#263, SchematicHQ/schematic-node#216).

json.dumps(ensure_ascii=False) matches the API except on U+2028 and U+2029, which the helper escapes afterwards. A Python string can hold a lone surrogate, or a surrogate pair as two separate characters, so the helper turns lone surrogates into U+FFFD and joins pairs, the way the API reads them. It raises ContextSignatureError for a key or value that isn't a string, and for a bool lifetime.

The tests run the shared vectors from the API (SchematicHQ/schematic-api#9729), copied into tests/context_signature/. Merge after #9729, and refresh the copy if the vectors change there.