[Refactor] Extract C2 logic into composable services

[google-labs-jules]

This PR refactors the tavern/internal/c2 package to use Composition over Inheritance. It extracts duplicate logic into small, focused service packages: validation, registration, taskctx, and transaction. This improves code readability and maintainability.

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PR created automatically by Jules for task 11200402250636169771 started by @KCarretto

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2337    ±0	2337    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#117	✅	2337	2337	0	0	0	0	not captured

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2337	0	0	43.8s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
eldritch: random::string_impl::tests::test_string_uniform	3	39.7s	43.8s
eldritch: random::string_impl::tests::test_string_uniform	3	39.7s	43.8s
eldritch: random::string_impl::tests::test_string_uniform	3	39.7s	43.8s
eldritch: process::info_impl::tests::test_info_default	3	2.7s	8.0s
eldritch: process::info_impl::tests::test_info_default	3	2.7s	8.0s
eldritch: process::info_impl::tests::test_info_default	3	2.7s	8.0s
eldritch: pivot::port_scan_impl::tests::test_portscan_return_type_starlark_dict_from_interpreter	3	2.5s	7.5s
eldritch: pivot::port_scan_impl::tests::test_portscan_return_type_starlark_dict_from_interpreter	3	2.5s	7.5s
eldritch: pivot::port_scan_impl::tests::test_portscan_return_type_starlark_dict_from_interpreter	3	2.5s	7.5s
eldritch: pivot::ssh_copy_impl::tests::test_pivot_ssh_copy	3	3.8s	7.4s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

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