Follow-ups to #290: quote config values in client-RX source warnings, pin message details, consistent disabled-coverage wording

[dborup]

Nits from the review of #290 (#278, merged as ecff198b). Details are in the review comment on #290.

1. N1 — log forgery. cmd/ingestor/client_rx_sources.go (~117) interpolates the allowlist entry with %s. A config value with an embedded newline can therefore split the warning and forge an extra log line. Use %q for every config-derived value in these warnings.

2. N2 — two surviving mutants in the new warning text:

   • strings.Join(ambiguous, "; ") changed to " ";
   • len(matched) replaced by the literal 2.

   Pin both with a stricter test.

3. N3 — contradictory wording. With clientRxCoverage.enabled: false, the state line says no coverage is ingested, while the new warning still speaks of coverage. Make them consistent.

4. N4 — duplicates repeated. A duplicated allowlist entry (["auth","AUTH"]) repeats the whole clause. Deduplicate case-insensitively before reporting.

[dborup]

Fixed by #326 (merged as 310c6caead804a0d5e721fc46a38e0f623daba14).