fix(sender): specific SIGHUP reload error messages

[andrescera]

What

Replaced generic parse-error logging in SIGHUP reload with specific, actionable messages for each failure mode:

• File not found/unreadable: "ips file not found/unreadable: , refusing reload"
• File empty: "ips file is empty: , refusing reload"
• Invalid IP lines: "invalid IP on line : skipping invalid lines in "
• Zero valid IPs: "Ignoring source IP reload from : no valid source IPs (parse error); keeping existing connections"

Why

Operators need clear diagnostics when SIGHUP reload fails. The previous generic "no valid source IPs (parse error)" message didn't distinguish between missing files, empty files, and garbage content, making troubleshooting harder.

How to verify

1. All 19 GTest suites pass: ctest in build/
2. New GTest cases cover each error path (8 new tests in test_sender_bootstrap)
3. sighup-reload.sh compat scenario passes: tests/compat/scenarios/sighup-reload.sh --build-dir build
4. No behavior change: reload guard still refuses zero-valid-IP reloads and keeps existing connections

Risks

None. This is a logging-only change; reload behavior and connection lifecycle are unchanged.

---

Checklist

• Docs updated if behavior or structure changed (Rule A: no behavior change, no doc updates needed)
• Started from updated main; branch rebased on latest canonical branch (Rule B)
• git grep -n '.omo' -- ':!.gitignore' returns nothing (Rule D)
• Tests pass; all 19 GTest suites green, sighup-reload.sh compat scenario passes