Fix inline fragments without a space before "on"

[dpep]

What

Client#parse's fragment-spread regex (/\.\.\.([a-zA-Z0-9_]+(::[a-zA-Z0-9_]+)*)/) matches ...on Type (no space) as a spread referencing a Ruby constant named On, since it only requires the identifier to immediately follow the three dots — it doesn't distinguish that from an inline fragment written without a space before on.

Both ...on Type { ... } and ... on Type { ... } are valid, equivalent GraphQL — whitespace between ... and on is optional per the spec. But only the spaced form parses correctly today; the unspaced form raises:

GraphQL::Client::ValidationError: uninitialized constant on

Why this is safe to fix with a lookahead

The GraphQL spec explicitly forbids naming a fragment on:

FragmentName : Name but not on

So ...on immediately followed by a word boundary can never be a legitimate fragment-spread reference — it's unambiguously an inline fragment. This PR adds a negative lookahead ((?!on\b)) to the regex to skip exactly that case, leaving every other fragment name (including ones that start with on, like onFragment, or literally named On with a capital O) unaffected.

Testing

Added test_client_parse_inline_fragment_without_space_before_on, which fails against master with the ValidationError above and passes with this fix. Ran the full test/test_client.rb suite locally — 17 runs, 0 failures, 0 errors (3 pre-existing skips, unrelated).