Endpoint to Datastar action converter

Converts ZIO HTTP Endpoint to Datastar action expressions (@get('/users/$id')).

• Method mapping (GET/POST/PUT/PATCH/DELETE), with Method.ANY fallback via OpenAPIGen.method
• Path params → $param placeholders
• GET query params → alphabetically sorted placeholders
• Minimal API: DatastarAction.asString(endpoint), DatastarAction.fromEndpoint(endpoint)

Also fixes: Body.fromCharSequence Scala 3.3.6+ compilation (charSequence.isEmpty → charSequence.length() == 0)

Files:

• zio-http-datastar-sdk/src/main/scala/zio/http/datastar/DatastarAction.scala (new)
• zio-http-datastar-sdk/src/test/scala/zio/http/datastar/DatastarActionSpec.scala (new)
• zio-http/shared/src/main/scala/zio/http/Body.scala (1-line fix)

/claim #3697