Record transport drops and attach client_report envelopes

[szokeasaurusrex]

Part of #882. Depends on #1002.

Record transport-side drops for queue overflow, rate-limit filtering, network failures, and HTTP status handling (2xx success, 413 + other non-429 4xx/5xx as send_error, no client report for 429). Attach pending client_report items to outgoing envelopes (piggyback only, no recursion) and apply behavior consistently across reqwest, curl, ureq, and embedded transports.

[linear]

RUST-156