chore: align base-cli dependency and CI pins to 0.4.3 #1999

Summary

Align Base with the released base-cli 0.4.3 provider without changing Base's own release version.

Scope

• update the development dependency pin in requirements-dev.txt;
• update every Base workflow source-checkout pin from v0.4.2 to v0.4.3;
• update workflow-contract tests and current documentation/context references that assert the old provider version;
• preserve the explicit source-checkout and installed-provider resolution policy.

Acceptance criteria

• no active Base dependency or CI pin refers to base-cli 0.4.2; historical changelog/review entries may remain historical;
• source-checkout tests consume the released v0.4.3 tag;
• focused workflow tests and the documented Base validation harness pass;
• no Base release is created.

Project fields

• Status: Backlog
• Priority: P1
• Area: CI
• Initiative: Dependency Alignment
• Size: S