Surface actual parse error in MergeYaml via Validated API

[timtebeek]

Summary

• When MergeYaml encountered a YAML parsing failure, the old maybeParse method silently discarded the ParseError result (via a Yaml.Documents instanceof filter), producing only a generic "Must be valid YAML" message
• Now extracts the actual error from the ParseExceptionResult marker and surfaces it through the Validated API, making failures diagnosable (e.g. SnakeYAML syntax errors, print-idempotence failures)
• Introduces validateParseable() returning Validated<Yaml> so that validate() uses the Validated API instead of exception-based control flow
• parse() delegates to validateParseable(), eliminating duplicated parsing logic

Test plan

• invalidYaml() test updated to match new validation message
• sourceNull() test still passes with existing "Must be valid YAML" message
• All existing MergeYamlTest tests pass

[timtebeek]

We saw this somehow pass validation but fail during execution, so rewrote the logic to capture and forward any parse failure message.

[MBoegers]

It feels strange to have an exception to signal failure. Failure is nothing special its expected in validation.
Also, creating an exception causes stack walking to fill the stacktrace and JIT deopt, maybe not very strong arguments in our case but still.

We could omit the exception and instead return the parsing error or erroneous and check with instaceof?

[MBoegers]

or turn the parse into a validateParseable method and return a validation result insteadt?

[timtebeek]

@MBoegers any chance you could review? Or should I ask someone else?

[MBoegers]

Interesting style to use the Validated<> API. I like!