fix: fail !env substitution when the named variable is missing (fixes #63)

[cpruijsen]

Summary

!env already recorded a parse error that named a missing variable, but parseYamlFile ignored doc.errors and returned the variable name as the value. validate / deploy then treated uri: PS_DATABASE_URI as a connection string, and Cloud reported PSYNC_S1109 (protocol must be postgresql, got undefined). Parse now fails with that existing message so the missing variable is named, and --validate-only=connections no longer swallows the failure.

The reporter reproduced the Cloud protocol error on #63. Docs and README only describe process environment variables, not .env files.

Decision

• Chose: Surface the existing !env parse error (name the missing variable). Do not auto-load .env / .env.local.
• Alternative: Load .env then .env.local from the working directory (without overriding exported variables).
• Why: The misleading error is a dropped parse failure, not missing dotenv. Auto-loading files is a new lookup policy this repo has never documented.
• Happy to switch to dotenv loading if that is preferred.

ensureServiceTypeMatches only reads _type, so it uses parseYamlDocumentPreserveTags and does not require template !env placeholders to be set (same idea as the docker plugin when merging snippets).

Testing

• Unit: parseYamlFile throws naming PS_DATABASE_URI when unset, and still substitutes when set.
• powersync validate --validate-only=connections fails with that name and does not call testConnection.
• powersync link self-hosted still works when service.yaml has unresolved !env URIs.

Closes #63

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[LucDeCaf]

Thanks for the contribution! Tested this on my side and it looks good, just a few small observations.
Also, don't forget to sign the CLA. :)

[LucDeCaf]

I noticed that when running powersync validate and powersync pull instance in the examples/cloud/basic-cloud-pull example, I got two differently formatted errors. This is preexisting and happens because only some calls to parseYamlFile are wrapped in try { ... } catch (error) { this.styledError(...); }. I'll make a followup PR to fix this; just mentioning it here.

[LucDeCaf]

This description is slightly misleading since the function now throws on any YAML parsing error, not just missing env vars.