Fix generate-schema format diagnostics

[mcc0nnell]

Align generate-schema --as help and invalid-format diagnostics with
ISchemaGenerator.SchemaFormat (JSON and XML only). YAML is no longer
advertised as a valid generated-schema format.

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Changes to Core Features:

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• Have you written new tests for your core changes, as applicable?
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What

Align generate-schema --as help and invalid-value diagnostics with the
formats actually supported by ISchemaGenerator.SchemaFormat.

Why

SchemaFormat currently supports only JSON and XML, but the error path
enumerates the broader databind Format enum and therefore advertises YAML.
The option help also describes --as as a source format and lists YAML.

Changes

• describe --as as the generated schema format
• derive valid choices from SchemaFormat.values()
• add regression tests for the help text and invalid YAML diagnostic

Validation

mvn -B -pl metaschema-cli -am -Dtest=GenerateSchemaCommandTest -Dsurefire.failIfNoSpecifiedTests=false test
Tests run: 2, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS
mvn -B -pl metaschema-cli -am -Prelease verify 
-Dtest=GenerateSchemaCommandTest -Dsurefire.failIfNoSpecifiedTests=false
Checkstyle: 0 violations
SpotBugs: 0
GenerateSchemaCommandTest: 2/2 passed
BUILD SUCCESS
Fixes #