fix(transport): Handle HTTP 413 with actionable log and use send_error for HTTP errors (#5115)

* fix(transport): Handle HTTP 413 with actionable log and use send_error for HTTP errors

Log a specific, actionable error message when Relay returns HTTP 413
(Content Too Large) instead of the generic "Request failed" message.
The message suggests reducing event/breadcrumb/attachment sizes and
mentions the `SentryOptions.onOversizedEvent` callback.

Also switch the client report discard reason for all HTTP 4xx/5xx
errors (except 429) from `network_error` to `send_error`, matching
the client reports spec and aligning with sentry-python and
sentry-cocoa.

Fixes GH-5050
Co-Authored-By: Claude <noreply@anthropic.com>

* docs: Add changelog entry for HTTP 413 handling

Co-Authored-By: Claude <noreply@anthropic.com>

* remove duplicate log message

* Update changelog for client report discard reason

Switch client report discard reason for HTTP 4xx/5xx errors.

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Co-authored-by: Claude <noreply@anthropic.com>