fix(validators): reject incomplete repository metadata - #1555

…rguments (#1583)

## Summary

Publish accepts arguments that violate the schema. `validateArgument`
only checks the named branch, so a `type` that is empty or outside the
enum passes through, and a positional argument with neither `value` nor
`valueHint` also passes. The scan in #1546 counted 52 entries with bad
argument types and 8 bare positional ones, my own paging of /v0/servers
found even more of the first kind. One I checked by hand:
`io.github.Jordan-Horner/symbols@1.0.1` (empty type under
packageArguments), another is `com.keyboardcrumbs/mcp@1.0.1` (type
"literal", still active). Every published schema version requires `type`
to be "positional" or "named" and positional args to carry `value` or
`valueHint`. Relates to #1546, the empty repository case is #1555's, and
version "latest" is already handled since #413.

## Change

`validateArgument` becomes a switch. Named branch unchanged. Positional
now requires `value` or `valueHint`. Unknown or empty `type` is
rejected. Two new errors in constants.go, references
`invalid-argument-type` and
`positional-argument-value-or-hint-required`.

Why not full schema validation at publish: the enhanced validation
design doc gates that behind the anyOf error consolidation (Phase 3),
raw anyOf failures on arguments come out as a noisy multi-error cluster.
These two checks give one clean message each and are one case arm each,
easy to delete at Phase 5 when schema-first lands. Until then publish
and edit just agree with what /v0/validate already reports.

Heads up on two consequences: editing a stored server that still carries
one of these legacy arguments will 422 until the argument is fixed, and
the importer skips invalid servers with a warning, so a re-seed from a
live dump drops those entries instead of failing. Also #1339 adds a
check inside the named branch of the same function, whichever lands
second has a small conflict to resolve there, happy to do it from my
side.

## Test

Tests first, they fail on main, the handler regression actually
publishes an empty-type argument with 200 on main and gets 422 with the
fix. On the branch: go test ./internal/... all green including the
handler tests against postgres, golangci-lint v2.13.1 (what CI pins) 0
issues, make validate passes. Two existing fixtures had positional args
with neither value nor valueHint, they got a valueHint since no
published schema version ever allowed that shape. New error paths and
references are pinned in validation_detailed_test.go.

## AI Disclosure

AI assistance (Claude) was used for issue research and during
development. All changes were reviewed and tested by the author.

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Co-authored-by: Radoslav Dimitrov <radoslav@stacklok.com>