Document all stable GraphQL validation error markers

[dkijania]

Context

#198 adds an important schema stability policy and names Cannot query field as stable validation/error text. Luis' latest review notes that downstream compatibility actually depends on three literal markers:

• Cannot query field
• Unknown argument
• Unknown type

mina-explorer-api matches all three to classify UpstreamSchemaError, drive tier fallback, and poison its capability cache. mina-explorer keys mostly on Cannot query field.

Follow-up work

• Update the policy/docs so all three literal markers are named as stable downstream contract, not only Cannot query field.
• Make it clear that changing any of the three messages, masking them, or replacing them with armor/cost-limit errors can be breaking.
• Consider explicit regression tests for Unknown argument and Unknown type alongside the existing Cannot query field checks.

Acceptance criteria

• docs/versioning.md or the schema stability policy names all three stable markers.
• The docs state that validation/parse masking changes affecting these markers require a major release.
• Regression coverage pins all three marker families, or the gap is explicitly accepted.

Related

• P2: Declare 1.0.0 + versioning & schema stability policy (#178) #198
• P0: Add GraphQL query-cost controls via graphql-armor (#164) #183
• P2: Verify error masking; make masking explicit (#177) #195