Address review findings on the auth/dpop-nonce posture:

- Record the token-request proof observation BEFORE the §8 nonce challenge,
  so a client that is challenged but does not retry is no longer mis-reported
  by token-request-proof as "never completed a token request" (its proof was
  just validated). (#2)
- Record the resource-request jti/replay on ANY valid proof (before the §9
  nonce gate), so fresh-proof is never asserted on zero evidence and a client
  that never honours the RS nonce fails ONLY rs-nonce. (modelcontextprotocol#5)
- Require challengeIssued && honored for as-nonce/rs-nonce SUCCESS, closing a
  vacuous-pass path (a client that pre-sends the nonce is never challenged). (#3a)
- Gate the AS nonce observation on the authorization_code exchange, matching
  recordTokenRequestProof (a refresh exchange must not satisfy §8). (#3c)
- Collapse duplicate shared token-flow check IDs (token-request, pkce-*) that
  the §8 round-trip re-POST would otherwise emit twice. (modelcontextprotocol#4)
- Example client: retry the token request only on a use_dpop_nonce error code,
  not any 400 carrying a DPoP-Nonce header (consistent with the RS path). (modelcontextprotocol#6)
- Fix the DpopTokenRequestObservation docstring ("last write wins" →
  sticky-failure). (modelcontextprotocol#7)

Adds an expectedSuccessSlugs option to the client test helper and tightens the
two nonce negative tests to pin the now-clean behavior (token-request-proof
SUCCESS for no-as-nonce; only rs-nonce fails for no-rs-nonce).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

- Refresh the held DPoP nonce from every response's DPoP-Nonce header
  (RFC 9449 §8.2 newest-wins) instead of capturing it once, so a server that
  rotates or single-uses its nonce no longer turns the negative probes into
  false "not testable" failures (or vacuous passes). (#1)
- Widen the stale iat probe to -303 s (matching the future side) so the ±1 s
  quantization of the whole-second Date header can't pull it onto the ±300 s
  boundary and be false-accepted. (#2)
- Update the isNonceChallenge comment and the untestable message: probes now
  carry the held nonce, so a use_dpop_nonce challenge is about the nonce
  lifetime (rotated/stale/single-use), not a missing nonce. (#3)
- Fixture: parse integer env vars NaN-safely (intEnv helper) so a malformed
  DPOP_CLOCK_OFFSET_SECONDS / DPOP_IAT_SKEW_SECONDS falls back to its default
  instead of silently disabling the iat window. (modelcontextprotocol#4)

Co-Authored-By: Claude Opus 4.8 <[email protected]>

- dedupeSharedChecks was masking failures: it kept the last occurrence of a
  duplicate check ID regardless of status and ran in BOTH postures, so a
  FAILURE on a challenged POST (or an earlier attempt in a restarted flow) was
  silently dropped. Replace with an exported, unit-tested collapseDuplicateChecks
  that prefers the MOST-SEVERE occurrence, and only apply it in the nonce
  posture (the baseline keeps genuinely distinct repeated attempts). (R5 #1)
- Reword the token-request-proof description: it can be SUCCESS for a client
  that presented a valid proof but never obtained a token (challenged, no
  retry), so it no longer claims "obtaining a DPoP-bound access token". (R5 #2)
- Reword the as-nonce/rs-nonce failure messages to cover a client that DID
  retry but whose retry proof was rejected (not only "did not retry"). (R5 #3)
- Document the deliberate replay decision: recording jti on challenged requests
  means re-sending the identical proof across the challenge/retry boundary trips
  replay detection — a genuine RFC 9449 §4.2 jti-uniqueness violation. (R5 modelcontextprotocol#5)

Adds src/scenarios/client/auth/dpop.test.ts unit-testing collapseDuplicateChecks
(prefers failures, keeps INFO, preserves order) — the dedupe fix is now pinned.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

- Refresh the held DPoP nonce from every response's DPoP-Nonce header
  (RFC 9449 §8.2 newest-wins) instead of capturing it once, so a server that
  rotates or single-uses its nonce no longer turns the negative probes into
  false "not testable" failures (or vacuous passes). (#1)
- Widen the stale iat probe to -303 s (matching the future side) so the ±1 s
  quantization of the whole-second Date header can't pull it onto the ±300 s
  boundary and be false-accepted. (#2)
- Update the isNonceChallenge comment and the untestable message: probes now
  carry the held nonce, so a use_dpop_nonce challenge is about the nonce
  lifetime (rotated/stale/single-use), not a missing nonce. (#3)
- Fixture: parse integer env vars NaN-safely (intEnv helper) so a malformed
  DPOP_CLOCK_OFFSET_SECONDS / DPOP_IAT_SKEW_SECONDS falls back to its default
  instead of silently disabling the iat window. (modelcontextprotocol#4)

Co-Authored-By: Claude Opus 4.8 <[email protected]>