Report a malformed class instance variable as a syntax error

[Halvanhelv]

Fixes #3148.

class M self. raises RuntimeError: Unexpected error instead of RBS::ParsingError, so it escapes rescue RBS::ParsingError and the CLI prints a raw Ruby backtrace. Every other malformed member in the same position reports correctly, and so does the same construct under interface:

input	before
class M <<<	RBS::ParsingError with location and caret
interface _I self.	RBS::ParsingError
class M self.	RuntimeError: Unexpected error
class M self.5	RuntimeError: Unexpected error

Cause

src/parser.c:2353, in the kSELF branch of parse_variable_member, passes false for the bool syntax_error parameter (include/rbs/parser.h:161). raise_error in ext/rbs_extension/main.c then discards the location, token and message and raises a bare RuntimeError:

if (!error->syntax_error) {
    rb_raise(rb_eRuntimeError, "Unexpected error");
}

49 other call sites in src/parser.c pass true and produce proper RBS::ParsingErrors.

Fix

Pass syntax_error with a specific message, in the wording used by the neighbouring sites.

The error is also set on next_token rather than current_token: the branch tests parser->next_token.type, so next_token is the offending token, and current_token would put the caret on the dot. Sibling sites that test next_token set the error on it the same way (lines 178, 288, 316, 325, 555, 665, 688, 704, 981).

Scope — why only this one site

src/parser.c has 12 other syntax_error = false sites. I checked them empirically rather than by eye: I tagged each with a unique marker, temporarily stopped main.c from discarding error->message, rebuilt, and ran targeted probes plus 25,000 mutation-fuzz iterations over the bundled signatures. Only 2353 was reached from user input. The rest are defensive default: arms over already-exhausted enums — e.g. the switch at 2383 covers exactly the four token types its caller dispatches on — so they indicate a parser bug, and RuntimeError is the correct semantic there. Left untouched.

Result

class M self.      1:13...1:13: unexpected token for class instance variable name, token=`` (pEOF)
module M self.     1:14...1:14: ... token=`` (pEOF)
class M self.5     1:13...1:14: ... token=`5` (tINTEGER)
class M self."x"   1:13...1:16: ... token=`"x"` (tDQSTRING)

CLI now prints a formatted error instead of a backtrace:

/tmp/bug.rbs:1:13...1:13: Syntax error: unexpected token for class instance variable name, token=`` (pEOF) (RBS::ParsingError)

  class M self.
               ^

Valid signatures are unchanged — self.@x, @x and @@x all still parse, and all 342 bundled core/, stdlib/ and sig/ files parse with no failures. Re-running the 25,000-input fuzz against the fixed build gives 23,649 RBS::ParsingError and 0 RuntimeError.

Tests

Added two tests to test/rbs/errors_test.rb: one asserting RBS::ParsingError across the four malformed forms, one checking the exact detailed_message and caret position. Both fail with RuntimeError(Unexpected error) without the fix.

Full suite before: 999 tests, 8198 assertions, 0 failures, 0 errors.
Full suite after: 1001 tests, 8204 assertions, 0 failures, 0 errors.