chore(coverage): stop adapter coverage from depending on packet timing

[yordis]

• Master CI failed the 100% coverage gate with every test passing, because some adapter branches only ran when the server's response happened to split across packets a certain way, or when DNS answered fast enough.
• A coverage gate that flips on scheduler luck reddens unrelated merges and hides real gaps, so each of those branches now has a test that pins the packet layout it needs.

[cursor]

PR Summary

Low Risk
Test-only changes; no production adapter logic is modified.

Overview
Makes Tesla adapter test coverage deterministic so CI’s 100% coverage gate no longer depends on DNS timing or how TCP splits HTTP responses.

Finch: The streaming error test now targets a closed local listener (:econnrefused) instead of nonexistent.invalid, so the failure path is exercised reliably.

Gun: Adds a test that forces Gun’s final {:gun_data, …, :fin, data} to arrive as separate :nofin and :fin messages via a reply_to splitter, covering plain-body assembly across multiple messages.

Mint: Adds a "responses split into scripted packets" suite that injects scripted {:tcp, socket, packet} sequences (headers-only, body split across packets, chunked vs plain, :stream / :chunks / max_body) so packet-boundary branches are hit without scheduler luck.

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[Copilot]

Copilot review overview

🟡 Changes recommended

Gun tests do not deterministically pin packet layout, so the targeted coverage branch may remain untested.

Review effort: Lite
Findings: 1

Open (1)

• Avoid sleeps; deterministically drive multi-part Gun messages · New

What changed in this PR

Stabilizes adapter coverage by making packet layouts and connection failures deterministic.

Changes:

• Adds scripted Mint packet tests.
• Adds Gun multi-part response coverage.
• Uses a refused loopback connection for Finch failure testing.

File	Reviewed changes
test/​tesla/​adapter/​mint_test.exs	Adds deterministic packet-layout tests.
test/​tesla/​adapter/​gun_test.exs	Adds multi-part body coverage, but relies on timing-sensitive sends.
test/​tesla/​adapter/​finch_test.exs	Replaces DNS-dependent failure testing.

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