feat(plugins): add provider request composition middleware (#511)

* feat(plugins): add provider request composition middleware

Expose per-attempt provider parameters and a final outbound preparation chain using middleware v6. Preserve native transport behavior when no outbound scope matches.

Co-authored-by: Codex <noreply@openai.com>

* fix(plugins): sync workbench with middleware v6

Co-authored-by: Codex <noreply@openai.com>

---------

Co-authored-by: Zyy <zyycn@users.noreply.github.com>
Co-authored-by: Codex <noreply@openai.com>

backend/apps/plugin-cli/tests/main.rs

@@ -250,7 +250,7 @@ fn source_manifest() -> &'static [u8] {
       "main":"bin/plugin",
       "runtime":"trustedProcess",
       "contributes":{
-        "middleware":{"version":5,"stages":["request"],"inputFormats":["openai"],"outputFormats":["openai"]},
+        "middleware":{"version":6,"stages":["request"],"inputFormats":["openai"],"outputFormats":["openai"]},
         "management":{"version":2}
       },