Feature/esp webpush v2

[zekageri]

## Summary

This PR cleans up the `esp-webPush` public transport API by renaming `Subscription` to `WebPushSubscription`, renaming `PushMessage.sub` to `PushMessage.subscription`, and removing app-level metadata fields (`deviceId`, `disabledTags`, and `deleted`) from the transport struct.

The motivation is to keep `esp-webPush` focused on Web Push transport concerns only, avoid collisions with generic `Subscription` types in consuming apps, and remove unused/non-transport fields from the library API.

Related issues: none linked.

## Type of Change

- [ ] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [x] Breaking change (fix or feature that would cause existing functionality to change)
- [x] Refactor (code change that neither fixes a bug nor adds a feature)
- [x] Documentation update
- [ ] Build/CI

## Details

This PR changes the public API and internal implementation to consistently use `WebPushSubscription` instead of `Subscription`.

It also removes fields that do not belong in the transport-layer type:
- `deviceId`
- `disabledTags`
- `deleted`

`PushMessage.sub` was renamed to `PushMessage.subscription` to match the new type name and make the API clearer.

`validateSubscription()` now validates only the required Web Push transport fields:
- `endpoint`
- `p256dh`
- `auth`

README examples, API reference, the example sketch, and the changelog were updated to reflect the breaking rename and field removals.

Reviewer context:
- This intentionally does not add tag filtering or any replacement metadata field.
- This intentionally does not provide a compatibility alias; downstream code must migrate to the new names.
- The goal is to narrow the library API to pure Web Push transport data and leave app-level recipient policy outside the library.

## Testing

- [x] Builds locally
- [x] Unit tests added/updated
- [ ] Tested on target hardware/device (if applicable)

Tested with local PlatformIO compile checks:

```bash
/home/zekageri/.platformio/penv/bin/pio ci /home/zekageri/projects/esptoolkit/libs/esp-webPush/test/test_esp_webPush \
  --board esp32dev \
  --lib /home/zekageri/projects/esptoolkit/libs/esp-webPush \
  --project-option platform=https://github.com/pioarduino/platform-espressif32.git \
  --project-option build_unflags=-std=gnu++11 \
  --project-option build_flags=-std=gnu++17 \
  --project-option lib_deps=ArduinoJson@>=7.0.0

/home/zekageri/.platformio/penv/bin/pio ci /home/zekageri/projects/esptoolkit/libs/esp-webPush/examples/basic_web_push \
  --board esp32dev \
  --lib /home/zekageri/projects/esptoolkit/libs/esp-webPush \
  --project-option platform=https://github.com/pioarduino/platform-espressif32.git \
  --project-option build_unflags=-std=gnu++11 \
  --project-option build_flags=-std=gnu++17 \
  --project-option lib_deps=ArduinoJson@>=7.0.0

Both completed successfully.

Unit tests were updated for the renamed API, and a transport-only validation case was added to confirm the struct now requires only endpoint, p256dh, and auth.

Compatibility / Breaking Changes

This is a breaking API change.

Changes:

• Subscription -> WebPushSubscription
• PushMessage.sub -> PushMessage.subscription
• Removed deviceId
• Removed disabledTags
• Removed deleted
• Removed deleted-state validation from validateSubscription()

Migration steps:

• Rename all usages of Subscription to WebPushSubscription
• Rename all usages of PushMessage.sub to PushMessage.subscription
• Remove any writes/reads of deviceId, disabledTags, and deleted
• Move app-level recipient metadata or filtering logic outside esp-webPush

Risks and Mitigations

Main risk:

• Downstream users will get compile errors because there is no compatibility alias.

Mitigations:

• README and example code were updated to the new API.
• Changelog includes an explicit breaking-change note.
• Tests were updated to cover the renamed API and transport-only validation behavior.

Secondary risk:

• Removing deleted changes preflight validation behavior.

Mitigation:

• This was intentional and documented; the transport struct now represents only Web Push credentials.

Checklist

• Code follows project style and conventions
• All tests pass locally
• Documentation updated (README, examples, comments)
• No secrets, credentials, or sensitive data committed
• Linked issue(s) and relevant context provided

Note: local compile checks passed, but I am not marking "All tests pass locally" because I did not complete a separate ESP-IDF smoke verification beyond the Arduino-side compile checks.

Environment

• Toolchain/SDK versions: PlatformIO pio ci, platform-espressif32 from https://github.com/pioarduino/platform-espressif32.git, ArduinoJson >= 7.0.0, C++17
• OS and version: Linux local development environment
• Other relevant environment details: validation performed against esp32dev target