Default Python workers to protocol 1.20

[rmcdaniel]

Summary

Part of durable-workflow/.github#180.

• Default worker headers and package declaration to protocol 1.20.
• Preserve explicit DURABLE_WORKFLOW_WORKER_PROTOCOL_VERSION=1.19, with startup compatibility checked against the selected protocol.
• Refuse unsupported worker protocol selections before registration, without silently lowering cooperative requests.
• Keep optional worker capabilities explicit and make ordinary CI exercise the SDK default.
• Cover default 1.20, explicit 1.19, older Server refusal and unchanged control-plane protocol. Positive cooperative worker cases use the default.

Validation and release

Ruff and mypy pass locally. Normal source and integration checks must pass before merge. Stable publication will carry this change onto the current stable package base without publishing the separate unqualified Nexus producer release candidate. Published-package and affected-consumer qualification remains tracked in the owning issue.