imap-proto: handle unknown FETCH attributes (RFC 8474 EMAILID/THREADID)

[x-hannibal]

Problem

RFC-compliant IMAP servers can include extension attributes in FETCH responses that imap-proto does not yet explicitly support. Known cases:

• RFC 8474 §5 OBJECTID: EMAILID, THREADID — sent by Apache James 3.9, Dovecot (with OBJECTID plugin), and others
• RFC 8514 §2: SAVEDATE
• Any future extension attribute

When one of these attributes appears in a FETCH list, msg_att's alt() exhausts all known parsers. Because nom 7's alt returns the last alternative's error (not the deepest), the caller sees a TakeWhile1 error from response_tagged's imap_tag — which tries to match * as a tag character and fails. The entire response is dropped.

Fix

Add msg_att_unknown as the last arm of msg_att's alt(). It consumes:

• a single-level parenthesised value (...) — covers EMAILID (Mxxx), THREADID (Txxx)
• an nstring — covers THREADID NIL, SAVEDATE "date-time"
• a bare atom or number — fallback for any remaining scalar value

and returns AttributeValue::Unknown so the rest of the FETCH attribute list can be parsed without error.

AttributeValue::Unknown is added to the enum as a documented variant. The name and raw value are consumed and discarded; callers that need the actual value for a specific extension should open a dedicated tracking issue or PR to add first-class support.

Tests

Four new tests in parser/tests.rs:

Test	Input
test_fetch_rfc8474_emailid	EMAILID (Mxxx) alongside UID + BODY[]<0>
test_fetch_rfc8474_threadid	THREADID (Txxx) alongside UID + BODY[]<0>
test_fetch_rfc8474_threadid_nil	THREADID NIL (RFC 8474 §5.2 allows NIL)
test_fetch_rfc8474_emailid_and_threadid	both in the same FETCH response

All 86 existing tests continue to pass.

Notes

• msg_att_unknown is intentionally the last alternative so it never shadows an explicitly supported attribute.
• Single-level parentheses only — deeply nested values (which do not appear in any known extension attribute grammar) are not handled. A dedicated parser can be added per RFC if needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.71%. Comparing base (6593414) to head (1c548e6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #193      +/-   ##
==========================================
+ Coverage   76.35%   76.71%   +0.36%     
==========================================
  Files          21       21              
  Lines        2829     2847      +18     
==========================================
+ Hits         2160     2184      +24     
+ Misses        669      663       -6     

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