Improve GroupByKey coder validation diagnostics

Three independent validation failures shared one message, and its formatting arguments were passed in the wrong order. Users could not reliably distinguish mismatched key coders, non-iterable output coders, or mismatched value coders.

Split the combined condition into focused validations with the relevant input and output coder identifiers or URN, then added regressions for valid coders and each failure mode.

• Shortens diagnosis of malformed portable pipelines by reporting the exact coder contract that failed.
• Prevents misleading error text caused by reversed formatting arguments.
• Locks in actionable validation behavior for key, iterable output, and value coder relationships.

• Added regression coverage for valid coders, key mismatches, invalid output URNs, and value mismatches; eight focused pipeline utility tests passed.
• The focused Fn API GroupByKey runner test and 90 CI checks passed; one wheel job reported only a Docker teardown failure after building and repairing its artifact.
• The authored commit and GitHub-verified merge credit Goutam Adwant; a Beam collaborator approved the change and GH-27929 closed on merge.