chore(tests): remove Langfuse scoring from preprocess tests

Scoring is only meaningful for LLM-backed steps. Preprocess tests are
deterministic Python, so scores add no value. Kept only for detect tests.

- test_preprocess: drop _lf_score call, simplify _run_assertions return type
- cases.yaml: remove score_name from all op=preprocess entries

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/fixtures/preprocessors/cases.yaml

@@ -66,7 +66,6 @@ cases:
 
   - id: "1.5"
     description: "Email: strip HTML tags"
-    score_name: preprocess.email_strip
     file: email_action.html
     op: preprocess
     input_content_type: email_html
@@ -77,7 +76,6 @@ cases:
 
   - id: "1.6"
     description: "Email: extract metadata (Subject + From)"
-    score_name: preprocess.email_metadata
     file: email_action.html
     op: preprocess
     input_content_type: email_html
@@ -86,7 +84,6 @@ cases:
 
   - id: "1.7"
     description: "Email: split thread — solo ultimo messaggio"
-    score_name: preprocess.email_thread
     file: email_thread.html
     op: preprocess
     input_content_type: email_html
@@ -96,7 +93,6 @@ cases:
 
   - id: "1.8"
     description: "Email: singolo messaggio senza thread"
-    score_name: preprocess.email_single
     file: email_single.html
     op: preprocess
     input_content_type: email_html
@@ -105,7 +101,6 @@ cases:
 
   - id: "1.9"
     description: "Email: HTML pesante con table layout"
-    score_name: preprocess.email_heavy_html
     file: email_heavy.html
     op: preprocess
     input_content_type: email_html
@@ -118,7 +113,6 @@ cases:
 
   - id: "1.10"
     description: "Fallback: file sconosciuto → testo restituito"
-    score_name: preprocess.fallback
     file: fallback.txt
     op: preprocess
     input_content_type: unknown