[#1] Add JWT claims validation with overflow-safe timestamp bounds

[lincai505011-ops]

/claim #1

What

Implement a JWT claims validation library with safe timestamp bounds checking to prevent time.Time overflow, as specified in the acceptance criteria.

Changes

• jwt/claims.go: Core library with:

  • ParseClaims() — extracts registered claims from raw JWT token
  • ValidateTimestamps() — checks exp/nbf/iat fall within safe Unix range (year 1 to year 9999)
  • Validate() — full validation including expiry and not-before checks with configurable leeway
  • Rejects NaN, Inf, negative overflow, and values beyond MaxUnixTime
  • Specific error types: ErrInvalidEpoch, ErrMalformedClaims, ErrExpired, ErrNotYetValid

• jwt/claims_test.go: 13 test cases covering:

  • MaxInt64 overflow → rejected
  • Large float (1e21) → rejected
  • Extreme negative → rejected
  • NaN and ±Inf → rejected
  • Valid future date (year 2050) → accepted
  • MaxUnixTime boundary → accepted
  • MinUnixTime boundary → accepted
  • Expired token → rejected with ErrExpired
  • Not-yet-valid token → rejected with ErrNotYetValid
  • Leeway (30s expired, 60s leeway → accepted)
  • All three claims overflow → rejected
  • Zero/unset claims → skipped
  • Full round-trip test

Test Output

=== RUN   TestParseClaims --- PASS
=== RUN   TestParseClaims_Invalid --- PASS
=== RUN   TestValidateTimestamps_Overflow --- PASS
=== RUN   TestValidateTimestamps_LargeFloat --- PASS
=== RUN   TestValidateTimestamps_Negative --- PASS
=== RUN   TestValidateTimestamps_NaN --- PASS
=== RUN   TestValidateTimestamps_Inf --- PASS
=== RUN   TestValidateTimestamps_Valid --- PASS
=== RUN   TestValidate_Expired --- PASS
=== RUN   TestValidate_NotYetValid --- PASS
=== RUN   TestValidate_Leeway --- PASS
=== RUN   TestValidateTimestamps_AllClaimTypes --- PASS
=== RUN   TestValidateTimestamps_Zero --- PASS
=== RUN   TestRoundTrip --- PASS
PASS
ok  	github.com/heathivorjocelyn6/jwt/jwt	0.041s

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