[yaml] Fix tag URI decoding, numeric tag RangeErrors, and dead checks - #2630

Rationale

1. Tag URI decoding & shorthand local tag suffixes (Scanner._scanTagUri): Scanner._scanTagUri previously ignored the StringBuffer holding the already-scanned tag head from _scanTagHandle, causing shorthand local tags (such as !custom 123) to be rewritten to the non-specific '!' tag instead of being rejected as undefined tags. Additionally, _scanTagUri passed unvalidated % sequences directly to Uri.decodeFull, leaking uncaught ArgumentError on malformed percent escapes (e.g. !<%>, !a!%, %TAG ! tag:%) and raw FormatException without a SourceSpan on non-UTF-8 percent-encoded sequences (e.g. !<%C3%28>).
2. Empty and 1-character !!int / !!float scalar RangeErrors (Loader._parseNumberValue): Empty scalars tagged with !!int or !!float (!!int "", - !!int\n, !!float "") threw RangeError on contents.codeUnitAt(0) instead of YamlException, and 1-character !!float scalars (!!float a, !!float ., or valid !!float 0) skipped the if (allowInt && length == 1) branch and threw RangeError on contents.codeUnitAt(1).
3. Unreachable parser/scanner checks: Parser defined unused _State.BLOCK_NODE_OR_INDENTLESS_SEQUENCE and _State.FLOW_NODE enum states, and Scanner._scanAnchor checked 5 delimiter characters already consumed by _isAnchorChar.

Summary of Changes

• pkgs/yaml/lib/src/scanner.dart:
  • Append the remaining scanned tag URI substring to buffer in _scanTagUri so shorthand local tag suffixes scanned in _scanTagHandle are preserved, and add _decodeTagUri / _isHexChar to validate %XX escapes and wrap FormatException in YamlException.
  • Simplify _staleSimpleKeys(), remove 5 unreachable delimiter checks in _scanAnchor(), and remove the redundant null check in _isStandardCharacterAt().
• pkgs/yaml/lib/src/loader.dart:
  • Guard if (length == 0) return null; and handle if (length == 1) for both allowInt and allowFloat in Loader._parseNumberValue.
• pkgs/yaml/lib/src/parser.dart:
  • Remove unused _State.BLOCK_NODE_OR_INDENTLESS_SEQUENCE and _State.FLOW_NODE enum states and redundant tagToken?.suffix fallback.
• pkgs/yaml/test/yaml_test.dart, pkgs/yaml/test/yaml_test_suite_test.dart, & pkgs/yaml/CHANGELOG.md:
  • Un-skip YAML 1.2 spec test [Example 10.7] (zero: !!float 0), add regression tests for tag URI decoding and empty/1-char !!int and !!float scalars, update _expectedFailures, and document the fixes under 3.2.0-wip.

Verification

• dart analyze --fatal-infos: exit 0
• dart format --output=none --set-exit-if-changed .: exit 0
• dart test -c source -r compact: exit 0 (595 tests passed)