fix: spawn network tasks detached

[eudelins-zama]

Description of changes

• Spawn network tasks detached, since the handles were only ever joined during shutdown (in Drop), which in practice means never.
• Remove the ThreadHandleGroup entirely from the codebase.

Issue ticket number and link

Closes https://github.com/zama-ai/kms-internal/issues/2929

PR Checklist

I attest that all checked items are satisfied. Any deviation is clearly justified above.

• Title follows conventional commits (e.g. chore: ...).
• Tests added for every new pub item and test coverage has not decreased.
• Public APIs and non-obvious logic documented; unfinished work marked as TODO(#issue).
• unwrap/expect/panic only in tests or for invariant bugs (documented if present).
• No dependency version changes OR (if changed) only minimal required fixes.
• No architectural protocol changes OR linked spec PR/issue provided.
• No breaking deployment config changes OR devops label + infra notified + infra-team reviewer assigned.
• No breaking gRPC / serialized data changes OR commit marked with ! and affected teams notified.
• No modifications to existing versionized structs OR backward compatibility tests updated.
• No critical business logic / crypto changes OR ≥2 reviewers assigned.
• No new sensitive data fields added OR Zeroize + ZeroizeOnDrop implemented.
• No new public storage data OR data is verifiable (signature / digest).
• No unsafe; if unavoidable: minimal, justified, documented, and test/fuzz covered.
• Strongly typed boundaries: typed inputs validated at the edge; no untyped values or errors cross modules.
• Self-review completed.

Dependency Update Questionnaire (only if deps changed or added)

Answer in the Cargo.toml next to the dependency (or here if updating):

1. Ownership changes or suspicious concentration?
2. Low popularity?
3. Unusual version jump?
4. Lacking documentation?
5. Missing CI?
6. No security / disclosure policy?
7. Significant size increase?

More details and explanations for the checklist and dependency updates can be found in CONTRIBUTING.md

[dvdplm]

LGTM

[dvdplm]

For future self, here's the link to the internal discussion around this: https://zama-ai.slack.com/archives/C05UD8KC0UD/p1772530437428579

[maksymsur]

LGTM

[github-actions]

Consolidated Tests Results 2026-03-09 - 19:08:47

Test Results

16 passed

Details

16 tests
not captured
junit-to-ctrf
build-and-test test-reporter #778
fix: spawn network tasks detached #454

test-reporter: Run #778

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Pending ⏳	Other ❓	Flaky 🍂	Duration ⏱️
16	16	0	0	0	0	0	not captured

🎉 All tests passed!

Tests

View All Tests

Test Name	Status	Flaky	Duration
nightly_full_gen_tests_k8s_default_threshld_sequential_crs	✅		33.0s
test_k8s_threshld_insecure	✅		3m 14s
k8s_test_crs_uniqueness	✅		33.0s
k8s_test_insecure_keygen_encrypt_and_public_decrypt	✅		3m 16s
k8s_test_insecure_keygen_encrypt_multiple_types	✅		3m 36s
k8s_test_keygen_and_crs	✅		3m 14s
k8s_test_keygen_uniqueness	✅		8m 54s
full_gen_tests_k8s_default_threshld_sequential_crs	✅		32.8s
test_k8s_threshld_insecure	✅		3m 16s
k8s_test_crs_uniqueness	✅		32.9s
k8s_test_keygen_and_crs	✅		3m 15s
k8s_test_keygen_uniqueness	✅		8m 52s
nightly_full_gen_tests_k8s_default_centralzd_sequential_crs	✅		1.7s
test_k8s_centralzd_insecure	✅		1m 4s
k8s_test_centralized_insecure	✅		1m 1s
nightly_full_gen_tests_default_k8s_centralized_sequential_crs	✅		1.7s

_{🍂 No flaky tests in this run.}

Github Test Reporter by CTRF 💚

🔄 This comment has been updated

[titouantanguy]

LGTM ! Congrats finding this :)