release(v3): [3/4] Remove dead error handling code

[lasith-kg]

📚 PR Train

This PR is part of a stacked PR train delivering the v3 release. Review/merge bottom-up in order:

#	PR	Change	Base
1	#170	Modernize repository to actions/typescript (ESM + Node 24)	main
2	#171	Add support for return_run_details	release/v3-1-node-24
3	👉 #172 (this PR)	Remove dead error handling code	release/v3-2-dispatch-workflow
4	#173	v3 release (combined → main)	main

⚠️ PRs 1–3 are stacked (each targets the one above). PR #173 is the combined branch that actually merges into main.

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Removes redundant HTTP status-code guards that could never fire — Octokit already rejects on any status >= 400, so those checks were dead code.

Highlights

• Removed unreachable status guards from repositoryDispatch, getWorkflowId, getWorkflowRuns, and getDefaultBranch.
• Kept (and documented) the one meaningful guard — workflowDispatch still throws on a legacy 204 response to avoid silently returning undefined run details.
• Tests updated to assert Octokit errors propagate rather than being swallowed.

No user-facing behavior change; failures now surface via Octokit's own (more descriptive) errors.