[AI] Make HostProcess principal field optional

[KCarretto]

Updated the HostProcess ent schema in tavern/internal/ent/schema/host_process.go to make the principal field optional and removed the .NotEmpty() validator. This change satisfies the requirement for issue #1758.

Key changes:

• Modified tavern/internal/ent/schema/host_process.go.
• Regenerated Ent and GraphQL code using go generate ./tavern/internal/ent.
• Verified that principal is now nillable and optional in the generated builders.
• Ran backend integration and unit tests (ent, c2, graphql) to ensure no regressions.

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PR created automatically by Jules for task 12980144447402907872 started by @KCarretto

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2949	2949	0	0	0	0	1ms

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#2099	✅	2949	2949	0	0	0	0	1m

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2949	0	0	3m 28s

Test Changes

0 test added, 0 removed

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
eldritch-libfile: std::list_recent_impl::tests::test_list_recent_default_args	3	2m 32s	3m 28s
eldritch-libfile: std::list_recent_impl::tests::test_list_recent_default_args	3	2m 32s	3m 28s
eldritch-libfile: std::list_recent_impl::tests::test_list_recent_default_args	3	2m 32s	3m 28s
TestDockerExecutor_Build_ContextCancellation	1	5.7s	5.7s
eldritch-libsys: std::dll_inject_impl::tests::test_dll_inject_simple	1	5.2s	5.2s
TestInteractiveShell	1	5.0s	5.0s
TestOtherStreamOutput	1	5.0s	5.0s
TestDockerExecutor_Build_SimpleEcho	1	3.3s	3.3s
imix::bin/imix: tests::task_tests::test_task_eprint_behavior	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s

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

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