refactor: make dev-server, login, and sourcemaps commands testable via CallCmd()

[zmdavis]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Describe the solution you've provided

Wire NewStartServerCmd, NewLoginCmd, and NewSourcemapsCmd through the existing APIClients struct instead of constructing hardcoded real clients inline. Add dev_server.MockClient and three tests for start_server.go.

This is the first step in a larger effort to improve test coverage in this repo.

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Note

Low Risk
Primarily dependency-injection and new unit tests; runtime behavior should be unchanged aside from how clients are constructed and passed in.

Overview
Refactors CLI wiring so login, dev-server, and sourcemaps commands use clients provided via cmd.APIClients rather than constructing real resources/dev_server clients inline, making these commands testable through cmd.CallCmd().

Adds a dev_server.MockClient and new dev-server start tests that assert --context and --override JSON are parsed and forwarded to RunServer, and that malformed context JSON returns an error without starting the server.

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