fix: frontmatter was adding newlines in some cases causing invalid model ids (#11095)

Co-authored-by: aptdnfapt <aptdnfapt@users.noreply.github.com>

packages/opencode/test/config/fixtures/markdown-header.md

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+# Response Formatting Requirements
+
+Always structure your responses using clear markdown formatting:
+
+- By default don't put information into tables for questions (but do put information into tables when creating or updating files)
+- Use headings (##, ###) to organise sections, always
+- Use bullet points or numbered lists for multiple items
+- Use code blocks with language tags for any code
+- Use **bold** for key terms and emphasis
+- Use tables when comparing options or listing structured data
+- Break long responses into logical sections with headings

packages/opencode/test/config/fixtures/weird-model-id.md

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+---
+description: General coding and planning agent
+mode: subagent
+model: synthetic/hf:zai-org/GLM-4.7
+tools:
+  write: true
+  read: true
+  edit: true
+stuff: >
+  This is some stuff
+---
+
+Strictly follow da rules