Description of Change / Summary

This PR makes TLS writes robust against partialmbedtls_ssl_write() returns and eliminates zero-length writes. It also sends large payloads in 4 KB chunks which significantly reducesMBEDTLS_ERR_NET_CONN_RESET (−0x0050) occurrences on long HTTP bodies (e.g. multipart uploads).

Motivation / Problem

Insend_ssl_data() we currently callmbedtls_ssl_write(ctx, data, len) once and return as soon as it returns >0. This means we may send less thanlen bytes and rely on higher layers to retry. Many callers (includingWiFiClientSecure::write) then return that partial size to user code, which can break long HTTP requests.
Additionally, zero-length writes were not guarded; we observed “Writing … 0 bytes” followed by peer resets during multipart uploads.

Fix

• Loop until theentire buffer is written; handleWANT_READ/WRITE correctly.
• Guard zero-length calls (len==0 -> return 0).
• Chunk large writes (4 KB), which proved more reliable with mbedTLS + some servers and access points.

Behavioral changes

• No API changes.WiFiClientSecure::write() still returns the number of bytes written, now reliably== size on success.
• Failure paths and timeouts remain unchanged.

Test Scenarios

• Reproduced with a Telegram Bot multipartsendPhoto upload. Before: frequent(-0x0050) MBEDTLS_ERR_NET_CONN_RESET mid-body, sometimes preceded by a 0-byte write; after patch: stable uploads.
• Verified on ESP32-S3; TLS handshake and certificate verification unaffected (same logs as before).
• Also verified small payloads and GET requests.

Why chunking?

Some setups are sensitive to very large TLS records and single-shot writes. Chunking to 4 KB reduces resets without measurable throughput loss and keeps memory usage predictable.

Alternatives

• Make chunk size configurable; we kept 4 KB static for simplicity.
• Do the loop inWiFiClientSecure::write() instead—this would duplicate logic; it’s more coherent inssl_client.cpp.

Risks

• Low. The change only affects the write path; handshake/verify/reads are untouched.

Logs (before / after)

• Before: header then “Writing HTTP request with 0 bytes…” →(-80) UNKNOWN ERROR CODE (0050) (peer reset).
• After: multiple 4 KB writes; no resets; server returns 200 with JSON body.