enhancement: Add [[nodiscard]] to BLE error-returning methods

[gdellis]

Context

BleServer methods that return esp_err_t can have their return values ignored silently. If a caller forgets to check the error, failures go undetected.

Suggested Fix

Add [[nodiscard]] attribute to methods returning esp_err_t:

[[nodiscard]] esp_err_t init ();
[[nodiscard]] esp_err_t start ();
[[nodiscard]] esp_err_t stop ();
[[nodiscard]] esp_err_t update_location (const BleLocationData& location);
[[nodiscard]] esp_err_t send_alert (const BleAlertData& alert);
[[nodiscard]] esp_err_t set_device_name (const char* name);

This causes compiler warnings when return values are ignored.

Priority

Low - defensive coding improvement

[gdellis]

The [[nodiscard]] attribute has already been added to all esp_err_t returning methods in ble.hpp via PR #52. This issue is resolved.