Bound Wi-Fi scan results to fixed storage

[Distortions81]

Summary

• Cap ESP-IDF scan retrieval at the existing 20-record storage limit.
• Pass explicit destination capacity through wifi_scan() and clamp both copy boundaries.
• Clear stale scan state on failure and terminate copied SSIDs.

Root cause and impact

The driver-reported AP total was reused as capacity for a fixed 20-entry global array, then reused for a fixed 20-entry HTTP stack array. A dense scan could write beyond both buffers. The endpoint still returns at most 20 networks; excess results are safely ignored.

This preserves the trusted-network design and adds no authentication or access control.

Related work

• Wifi scan fix #690 previously corrected Wi-Fi scan behavior; this draft adds the missing fixed-capacity enforcement.
• HTTP Handler Audit #1759 audited HTTP handlers; the second affected boundary is in the scan response handler.

Validation

• git diff --check upstream/master...HEAD
• ESP-IDF 6.0.2 connect component and ESP32-S3 test image built
• Source review against the ESP-IDF scan API capacity contract
• Current GitHub checks are green

A hardware scan with more than 20 visible APs remains desirable.

[github-actions]

Test Results

  2 files    2 suites   1s ⏱️
120 tests 120 ✅ 0 💤 0 ❌
122 runs  122 ✅ 0 💤 0 ❌

Results for commit e541ddf.