refactor(lading): annotate intentional-panic network generators

[goxberry]

Summary

Annotate intentional-panic .expect() sites in the network protocol generators with fn-level #[expect(clippy::expect_used, reason = "...")]. Reasons distinguish documented-contract panics from user-input parsing failures (the latter carry FIXME markers tracking eventual conversion to Error variants).

Sites annotated:

• `generator/tcp.rs::new` (FIXME — user-supplied addr parsing)
• `generator/grpc.rs` — gRPC client construction
• `generator/http.rs` — HTTP client + header parsing
• `generator/kubernetes/resource.rs` — k8s api construction
• `generator/procfs.rs` — sysfs path setup
• `generator/splunk_hec.rs` — Splunk endpoint setup
• `generator/splunk_hec/acknowledgements.rs::send` — channel routing invariant

Stacked on #1898. Part of the broader push toward enforcing clippy::expect_used = "deny" workspace-wide.

Test plan

• `./ci/validate` passes
• CI green on this PR

[goxberry]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(lading): surface tooling/FUSE-mount panics as errors #1904
• fix(lading): surface remote-input parsing failures as errors #1903
• fix(lading): surface user-config parsing failures as Error variants #1902
• refactor(lading): drop expect_used quarantines and annotate bins #1901
• refactor(lading): annotate intentional-panic observer/target sites #1900
• refactor(lading): annotate intentional-panic network generators #1899 👈 (View in Graphite)
• refactor(lading): annotate intentional-panic file_gen sites #1898
• refactor(lading): annotate intentional-panic blackhole + small generator sites #1897
• refactor(lading): convert structural-infallible .expect() to unreachable!() #1896
• refactor(capture): retire .expect() sites and drop quarantines #1894
• refactor(payload): retire remaining .expect() sites and drop quarantine #1893
• fix(payload): reject zero-byte arbitrary input for Block #1892
• refactor(payload): annotate intentional-panic .expect() sites #1891
• refactor(payload): mark in-function invariants as unreachable!() #1890
• refactor(payload): mark fallible-context infallibles as unreachable!() #1885
• refactor(payload): mark infallible .expect() sites as unreachable!() #1884
• chore(clippy): drop expect_used quarantine for lading_throttle #1883
• chore(clippy): forbid .expect() in production code #1882
• main

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