bug(fxconfig): config validation for NotificationsConfig does not call EndpointServiceConfig.Validate

## Description
In `tools/fxconfig/internal/config/config.go`, both `NotificationsConfig` and `QueriesConfig` embed `EndpointServiceConfig` via `mapstructure:",squash"` but neither of them implement their own `Validate()` method.
Because they lack a `Validate()` method, when `Provider.Get()` calls `p.cfg.Validate()` (in `provider.go`), it silently falls back to calling the embedded `EndpointServiceConfig.Validate()`.
While this validates the embedded fields, it completely skips validating the struct-specific fields. For example, `NotificationsConfig` has an additional `WaitingTimeout` field. Because this field is never validated, a user can set `waitingTimeout: 0s` or a negative value, and the configuration will silently accept it. This can result in the notification client being configured with invalid timeouts, leading to immediate transaction waiting failures.
### Expected Behavior
`NotificationsConfig` and `QueriesConfig` should have their own explicit `Validate(v validation.Context) error` methods that:
1. Delegate to the embedded `EndpointServiceConfig.Validate()` to validate shared fields.
2. Validate their own specific fields (e.g., ensuring `WaitingTimeout` is a positive duration within a reasonable upper bound, like max 10 minutes).
### Steps to Reproduce
1. Configure `fxconfig` with a `NotificationsConfig` where `waitingTimeout` is set to `-1s` or `0s`.
2. Run the application. The validation phase passes without errors.
3. The notification client receives an invalid timeout parameter, leading to unexpected failures during transaction waiting.
### Proposed Solution
Add explicit `Validate` methods for `NotificationsConfig` and `QueriesConfig` inside `tools/fxconfig/internal/config/validate.go`. Add unit tests ensuring that negative or excessively large duration values are correctly rejected.