Remove unused TLS material loader and update tests (issue #345)

[octoaide]

Closes #345

Summary

This PR removes an unused thin wrapper function, load_tls_material,
from src/main.rs that triggered a dead-code warning on the default
cargo check path. Tests and documentation that previously referenced
that wrapper were updated to use load_tls_material_with_bytes
directly and discard the raw TLS bytes where only parsed Certs are
required.

What changed

• Removed load_tls_material from src/main.rs.
• Updated rustdoc for load_tls_material_with_bytes to remove references
  to the removed wrapper.
• Updated tests (including the SIGHUP acceptance test in
  src/subscribe/tests.rs) to call load_tls_material_with_bytes and
  ignore the raw bytes when appropriate.
• Preserved existing validation behaviour: cert/key/CA material is still
  parsed and validated before being published to reload paths.

Files touched (high level)

• src/main.rs
• src/subscribe/tests.rs

Validation performed

• cargo fmt --all --check (passes)
• cargo check (no dead-code warnings)
• cargo check --tests (passes)
• cargo test load_tls_material (relevant tests pass)
• cargo test tls_reload (relevant tests pass)
• cargo test sighup_rerun_rebuilds_shared_endpoint_for_ingest_and_publish
  (passes)
• cargo clippy --tests --all-features (no warnings)

Notes

This is an internal cleanup with no user-visible behaviour changes.
When reviewing, please note PR #303 (cooperative shutdown work) so the
maintainer can ensure no overlap with concurrent changes.

Issue: #345

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.10%. Comparing base (1f8a0c0) to head (3f182c4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
- Coverage   89.11%   89.10%   -0.01%     
==========================================
  Files           7        7              
  Lines        3270     3269       -1     
==========================================
- Hits         2914     2913       -1     
  Misses        356      356              

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[danbi2990]

Looks good to me. I verified that the unused wrapper is removed, tests now call load_tls_material_with_bytes directly, and the TLS material validation behavior is preserved.