feat(manifest): expand env vars in foundational-store input

[maoueh]

Summary

Adds environment variable expansion ($VAR / ${VAR}) for the foundational-store module input in substreams.yaml. This lets a manifest be authored/pushed with a placeholder that is resolved at pack/load time:

modules:
  - name: my_module
    inputs:
      - foundational-store: $DEPLOYMENT_ID

This is a convenience at read time only — the generated .spkg always embeds the resolved, hard-coded value, never the $VAR reference.

Behavior

• Variable present (even empty) → its value is used.
• Variable absent → hard error listing the missing name(s), instead of silently substituting an empty string.

The same stricter (error-on-undefined) behavior is now also applied to the pre-existing expansion of imports and protobuf.importPaths, which previously used os.ExpandEnv (silent empty substitution).

Changes

• New strict expandEnvVars helper (os.Expand + os.LookupEnv, errors on undefined).
• Wired into expandManifestVariables (main Convert pipeline) for imports, protobuf import paths, and the new foundational-store inputs.
• Mirrored in the standalone LoadManifestFile helper for consistency.
• Docs: new "Environment variables" section in the manifest reference, with a caveat about the packaged .spkg embedding the resolved value.
• CHANGELOG updated (Added + Changed).

Tests

• Strict expander: present / present-empty / absent (single + multiple missing) / no-$.
• foundational-store expansion: happy path + undefined-var error path.

⚠️ Behavior change: undefined env vars in imports / protobuf.importPaths now fail loud instead of expanding to an empty string.