[w3c/ServiceWorker] Need for a mechanism to detect supported conditions and sources in the SW Static Routing API (Issue #1793)

yoshisatoyanagisawa created an issue (w3c/ServiceWorker#1793)

The Service Worker Static Routing API defines routing rules using a dictionary. This design presents a potential issue for developers. If a rule includes a condition that the user agent does not recognize or support, it may be silently ignored rather than causing a syntax error. This could lead to the rule being applied in unintended situations, creating unpredictable behavior that is difficult to debug.

For example, if a developer includes a new or experimental condition in a rule, the rule might be partially applied on browsers that don't support that specific condition, effectively ignoring a critical piece of the intended logic. This behavior makes feature detection and graceful degradation difficult to implement reliably.

To address this, I propose the introduction of a mechanism to allow developers to verify which conditions and sources are supported by the user agent. This could be achieved in a few ways:

1. **An API to Enumerate Supported Features**: Introduce a method that returns a list of supported condition and source values. For example:
```
const capabilities = await navigator.serviceWorker.getStaticRoutingCapabilities();
console.log(capabilities.supportedConditions); // ['urlPattern', 'requestMode', ...]
console.log(capabilities.supportedSources); // ['fetch', 'cache', ...]
```
2. **An API to Validate a Rule**: Provide a method that allows developers to check if a set of conditions or sources is supported before adding the rule. This would enable proactive validation.
```
// Returns true if all specified condition keys are supported by the UA.
const areConditionsSupported = await navigator.serviceWorker.supportsConditions([
  'requestMethod', 
  'or', 
]);

if (areConditionsSupported) {
  // All conditions are supported, so we can safely add the rule.
  await registration.routing.addRoutes({
    condition: { 
      or: [
        { requestMethod: "GET" },
      ]
    },
    source: "network"
  });
} else {
  // A condition is not supported. Use fallback logic.
  console.log('A required routing condition is not supported. Using a simpler rule.');
}
```
3. **A "Required" or "Mandatory" Field in `addRoutes()`**: Add an option to specify that certain conditions are essential for a rule. If a mandatory condition is not supported by the user agent, the `addRoutes()` call would reject, rather than silently ignoring the condition.
```
await registration.routing.addRoutes([{
  condition: {
    or: [
      { requestMethod: "GET" }
    ]
  },
  source: "network"
  // This would cause the call to fail if `requestMethod` is not supported.
  requiredConditions: ["requestMethod"]
}]);
```

This topic has been previously discussed in https://github.com/WICG/service-worker-static-routing-api/issues/28.

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