Update flagd-testbed to v3.6.0 and fix evaluation edge cases

[toddbaert]

Summary

Update the flagd-testbed dependency to v3.6.0 and fix the Go in-process evaluator to pass all new edge-case tests.

Testbed v3.6.0 adds coverage for the edge cases identified in:

• [BUG] sem_ver operator parsing behavior differs across SDK implementations #1873 (sem_ver parsing inconsistencies)
• [BUG] Custom operator error return values are inconsistent across SDK implementations #1874 (custom operator error return values)
• [BUG] $evaluators/$ref resolution: no parse-time validation, non-deterministic ordering, and missing Rust support #1875 ($evaluators/$ref resolution gaps)

Bugs to fix

sem_ver (#1873)

1. sem_ver returns false on parse failure instead of null; should return nil to trigger default variant fallback, not take the false branch
2. sem_ver returns false on unknown operator instead of null; same issue as above

Custom operator error returns (#1874)

3. ends_with returns false on error while starts_with returns nil; both should return nil consistently
4. fractional returns "" (empty string) on no-bucket-match; should return nil to trigger default variant fallback instead of causing a variant lookup failure

$evaluators/$ref resolution (#1875)

5. Non-deterministic $evaluators replacement ordering due to Go map iteration randomization; should use sorted or deterministic iteration
6. No diagnostic feedback on unresolved $refs at parse time; misconfiguration only discovered at evaluation time with confusing error about unknown $ref operator

Parent: #1904