[create-cloudflare] Enable upload_source_maps by default for new projects

[penalosa]

Supersedes #11726.

Instead of adding upload_source_maps: true to each of the 43 individual template wrangler.jsonc files, this sets it centrally in the updateWranglerConfig function — the same place that already sets name, compatibility_date, observability, and nodejs_compat for all C3 templates.

Like nodejs_compat, upload_source_maps is skipped for Python projects.

Key review points

• TOML vs JSON behavior difference: For TOML configs, ??= is used (won't override if already set). For JSON configs, appendJSONProperty (which calls assign) is used (will override). This matches the existing pattern for observability vs nodejs_compat but is worth verifying is intentional.
• Python exclusion: upload_source_maps is grouped with nodejs_compat inside the lang !== "python" guard. The original PR Set upload_source_maps to true for all C3 templates #11726 also removed it from Python templates explicitly.

Link to Devin session: https://app.devin.ai/sessions/9aa9987751da4a5b8ed3cf97a28996f5
Devin PR requested by @penalosa

---

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: docs already cover source maps at https://developers.cloudflare.com/workers/observability/source-maps/

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commit: b5685ee