fix: create-and-preview-style prompt now defaults to Standard, not streets-v12

[mattpodwysocki]

What changed

create-and-preview-style's base_style fallback and its "Create the map style" step now default to Standard, matching StyleBuilderTool's own long-standing policy ("ALWAYS use 'standard' as the default for all new styles"), instead of hardcoding the Classic streets-v12.

Why

Noticed while checking whether mapbox/mcp-server#248/#250 (switching render_map_tool to Standard, adding base-map restyling) implied any follow-up in this repo. They didn't — StyleBuilderTool and GeojsonPreviewUIResource were already Standard-first — but that check surfaced this prompt as a pre-existing, unrelated straggler that never got updated when StyleBuilderTool adopted its "always default to standard" policy.

It wasn't just the default string that was stale: the prompt's "Create the map style" step told the LLM to use the low-level create_style_tool directly, with a hand-authored example JSON skeleton (sources/layers background fill) hardcoded in the prompt text — a bare Classic-style structure, completely disconnected from the base_style argument (it never referenced it). That example would have been structurally wrong for a Standard-based style regardless of what the default was, since Standard needs the imports/config structure style_builder_tool already knows how to build. So the fix also switches the prompt to the same style_builder_tool → create_style_tool two-step pattern build-custom-map and other prompts in this repo already use, rather than patching the default string on top of a hand-rolled example that was already wrong.

Verification

• Added two tests: default resolves to "standard" (not "streets-v12") and references style_builder_tool; an explicit Classic base_style override is still respected.
• Existing test suite required no changes — none of it asserted step numbers or the old hardcoded JSON.
• npx tsc --noEmit, npx eslint: clean.
• Full suite passes (613/613).

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