Add handling for extra fields in from_dict method to solve platforms.webhook.port doesn't work problem

What does this PR do?

Fix a silent config drop bug where top-level keys written under a platform
block in config.yaml (e.g. platforms.webhook.port) were silently ignored
instead of being forwarded to the adapter.

Problem

PlatformConfig.from_dict() only recognised a fixed set of known fields
(enabled, token, api_key, home_channel, reply_to_mode, extra).
Any other key — such as port, host, secret, or routes — was silently
dropped when the YAML block was parsed.

Platform adapters (e.g. WebhookAdapter) read their settings exclusively from
config.extra:

# gateway/platforms/webhook.py
self._port = int(config.extra.get("port", DEFAULT_PORT))

So a user writing the natural top-level form:

# config.yaml
platforms:
  webhook:
    enabled: true
    port: 9000        # ← silently dropped, DEFAULT_PORT (8644) used instead

…would never see the port change take effect. The only working form was the
nested extra: block, which is not obvious from the documentation or the
error log (no warning is emitted):

platforms:
  webhook:
    enabled: true
    extra:
      port: 9000      # ← works, but not user-friendly

The same silent drop affected every other platform adapter that reads
settings from config.extra (e.g. api_server.host, api_server.key,
webhook secret, routes, etc.).

Related Issue

#10206

Fixes #

Type of Change

• [ ✅] 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

In PlatformConfig.from_dict() (gateway/config.py), after extracting the
known fields, iterate over the remaining keys and merge them into extra via
setdefault (so an explicit extra: sub-dict always takes precedence):

_KNOWN_KEYS = {"enabled", "token", "api_key", "home_channel", "reply_to_mode", "extra"}
extra = dict(data.get("extra", {}))
for key, value in data.items():
    if key not in _KNOWN_KEYS:
        extra.setdefault(key, value)

This means both config styles now work identically:

# Style 1 — top-level (now works)
platforms:
  webhook:
    enabled: true
    port: 9000

# Style 2 — nested extra (always worked, still takes precedence)
platforms:
  webhook:
    enabled: true
    extra:
      port: 9000

How to Test

# Start gateway with top-level port in config.yaml:
#   platforms:
#     webhook:
#       enabled: true
#       port: 9100
# Verify the server binds on 9100 instead of the default 8644.
python -m pytest tests/gateway/ -q