Fix: Support fallback values for ${param:x, fallback} syntax in stage parameters

[yashptel]

Summary

This PR restores support for default/fallback values in ${param:...} variable resolution syntax using the comma-separated fallback notation:

${param:some_non_existent_parameter, 'subnet-0123456789'}

Previously, this behavior was supported in Serverless Framework (especially via Compose), but recent versions of the OSS fork began throwing errors when the primary param could not be resolved.

Issue

Given this input:

provider:
  environment:
    sample_ecr_subnet: ${param:some_non_existent_parameter, 'subnet-0123456789'}

Users encountered:

Error: The param "some_non_existent_parameter" cannot be resolved from stage params...

This breaks compatibility with [the documented variable syntax](https://www.serverless.com/framework/docs/guides/variables#default-values) and behavior users depend on, especially when running through Compose.

Fix

• Updated param resolution logic to support fallback values when primary param is not defined.

• Removed hard-coded error throw on missing param and allowed fallback substitution.

• Fixes Default values (${x,y} syntax) don't work if param x is not defined #56

• Added tests:

  • Ensures fallback is used when the param is missing.
  • Confirms normal substitution when the param is available.
  • Verifies the new error code MISSING_VARIABLE_RESULT for unsupported formats.

[mnapoli]

Thanks!