fix: restore legacy X-Ray-Serve-Request-Id header for sticky routing

The ModelScope gateway (www.modelscope.cn/twinkle) requires the
`X-Ray-Serve-Request-Id` header name for sticky session routing.
Commit dff3143 replaced it with `x-request-id`, which Ray Serve 2.55+
reads natively but the gateway does not forward correctly.

Add `H_REQUEST_ID_LEGACY` alongside `H_REQUEST_ID` so both header names
are emitted, fixing `BadRequestError: Missing X-Ray-Serve-Request-Id
header` when connecting through the ModelScope gateway.

Author: Yunnglin

src/twinkle_client/http/headers.py

@@ -13,6 +13,7 @@
 
 # -- request id --
 H_REQUEST_ID = 'x-request-id'
+H_REQUEST_ID_LEGACY = 'X-Ray-Serve-Request-Id'
 
 # -- multiplexed model id (sticky routing) --
 H_MULTIPLEX = 'serve_multiplexed_model_id'
@@ -22,7 +23,7 @@
 H_AUTH = 'Authorization'
 H_AUTH_TWINKLE = 'Twinkle-Authorization'
 
-_ROUTING_HEADERS = (H_REQUEST_ID, H_MULTIPLEX, H_MULTIPLEX_LEGACY)
+_ROUTING_HEADERS = (H_REQUEST_ID, H_REQUEST_ID_LEGACY, H_MULTIPLEX, H_MULTIPLEX_LEGACY)
 _AUTH_HEADERS = (H_AUTH, H_AUTH_TWINKLE)