feat: Migrate synthetic message detection to metadata-based approach

[ananas-viber]

Summary

Currently, copilot.ts detects synthetic user messages (compaction, tool attachments, subtasks) via regex pattern matching on message text. This works but has maintenance coupling.

Current State

• synthetic: boolean already exists on TextPart (message-v2.ts:64)
• It's already being set in various places (prompt.ts:1374, etc.)
• BUT toModelMessage() does not propagate this flag to the final UIMessage

Proposed Change

1. In message-v2.ts toModelMessage():

   • Pass providerMetadata: { synthetic: true } for user text parts when part.synthetic === true
   • Add same metadata for compaction/subtask generated messages

2. In copilot.ts:

   • Check part.providerMetadata?.synthetic === true instead of pattern matching
   • Remove SYNTHETIC_PATTERNS array

Benefits

• Type-safe detection (no string coupling)
• Compile-time enforcement if field is renamed
• Cleaner architecture
• Eliminates theoretical "hackability" of pattern matching

Context

This is a follow-up to #8721 which implements the pattern-matching approach as a minimal fix.

Related issues:

• #8030 - Original report of excessive premium request consumption
• #8700 - Describes synthetic user messages burning premium requests (same root cause)
• #8721 - Pattern-matching fix (immediate solution)