chore(curl): Record envelope losses in the Curl transport

[szokeasaurusrex]

Record lost envelopes in the curl transport after the transport thread accepts them for sending. Curl execution failures are recorded as network_error; response-code lookup failures and non-429 HTTP 4xx/5xx responses are recorded as send_error.

Keep existing rate-limit handling for 429 responses and the existing payload-too-large debug message for 413 responses.

Closes #1152
Closes RUST-227

[linear-code]

RUST-227

[lcian]

LGTM

[sentry]

Bug: The code records an envelope loss and then immediately panics via .expect(), crashing the background transport thread and preventing future envelope sends.
_{Severity: MEDIUM}

Suggested Fix

The pattern of recording a loss and then panicking is inconsistent. The .expect() should be removed to allow the envelope to be dropped gracefully after the loss is recorded. This would prevent the transport thread from crashing and allow the application to continue attempting to send future events. Alternatively, if a panic is desired, the loss recording should be removed as it is redundant.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: sentry/src/transports/curl.rs#L123-L129

Potential issue: The code in the curl transport has a logical inconsistency for handling
envelope serialization failures. In the event of a `to_writer()` failure, the code first
uses `inspect_err()` to record the envelope loss with `LossReason::InternalError`, but
then immediately calls `.expect()`, which causes a panic. This panic occurs within a
detached background transport thread that has no panic recovery mechanism. Consequently,
the thread will crash silently, which stops all subsequent envelopes from being sent for
the remainder of the application's session. While serialization failure is rare, this
handling is problematic because it records a loss and then terminates the transport.

Also affects:

• sentry/src/transports/reqwest.rs:128~133