feat(mdns): post event when hostname changes

[zwx1995esp]

Summary

• Add the public mdns_hostname_changed_cb_t callback type and mdns_register_hostname_changed_callback() API.
• Notify registered callbacks after the hostname changes, including automatic hostname conflict resolution.
• Support multiple callback registrations and ignore duplicate (cb, arg) registrations.
• Keep hostname-change notification independent of the system event loop; callbacks receive the new hostname directly and must not call mDNS APIs.

Tests

• Add Linux host-unit coverage for callback registration, initial hostname assignment, unchanged hostname, and subsequent hostname changes.
• Add ESP target unit coverage for invalid registration arguments and hostname-change notification behavior.

Validation

• Linux host-unit test_receiver: 4 tests passed, including test_hostname_changed_callback.
• Built and flashed the M5Stack Thread Border Router example with the local mDNS component. The example now uses the callback API, and hostname updates were confirmed on the connected boards.

[david-cermak]

LGTM overall, the only thing is posting to the system/defautl event_post

Let's rework it for now to callbacks, something like this:

typedef void (*mdns_hostname_changed_cb_t)(const char *hostname, void *arg);
esp_err_t mdns_register_hostname_changed_callback(mdns_hostname_changed_cb_t cb, void *arg);

since this method is widely used alrady in the mdns component.

I'll think about the event loop usage and will probably create a private event loop that could be used instead of callbacks. BUt will do that in the next release

[david-cermak]

@zwx1995esp Also please rebase and resolve conflicts

[zwx1995esp]

LGTM overall, the only thing is posting to the system/defautl event_post

Let's rework it for now to callbacks, something like this:

typedef void (*mdns_hostname_changed_cb_t)(const char *hostname, void *arg);
esp_err_t mdns_register_hostname_changed_callback(mdns_hostname_changed_cb_t cb, void *arg);

Hi @david-cermak Thanks for your reviewing. I’ll refactor the related logic and rewrite the test cases. Once everything is ready, I’ll push the updated changes and let you know.

[zwx1995esp]

@david-cermak Refactor done, PTAL.

[david-cermak]

LGTM otherwise