Record HTTP send losses in the embedded-svc transport

[szokeasaurusrex]

Update EmbeddedSVCHttpTransport to record losses for HTTP-level send failures where the platform transport exposes enough information. Request construction/write/submit/read failures should be recorded as send_error; HTTP 413 and other non-429 4xx/5xx responses should also be recorded as send_error; 429 should not record a client-report loss for that response.

Scope

• Embedded HTTP send failures record losses for the attempted envelope.
• 413 responses record send_error losses and keep the existing debug message.
• Other non-429 4xx/5xx responses record send_error losses when status is available.
• 2xx responses and 429 responses do not record send losses.

[linear-code]

RUST-229