Cap control socket payload size

[lblommesteyn]

Summary

• cap the control/metrics Unix socket JSON payload size at 1 MiB so untrusted clients can’t force arbitrarily large allocations
• add a checked u64 → usize conversion so bogus length prefixes fail fast with InvalidInput
• cover the new behaviour with oversized_messages_are_rejected

Testing

• cargo +1.88.0 test -p ntpd oversized_messages_are_rejected

[codecov]

Codecov Report

❌ Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.49%. Comparing base (528e142) to head (e43a118).

Files with missing lines	Patch %	Lines
ntpd/src/daemon/sockets.rs	84.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2007      +/-   ##
==========================================
- Coverage   83.50%   83.49%   -0.02%     
==========================================
  Files          70       70              
  Lines       19315    19339      +24     
==========================================
+ Hits        16129    16147      +18     
- Misses       3186     3192       +6     

Flag	Coverage Δ
fuzz-cookie_parsing_sound	0.41% <ø> (-0.05%)	⬇️
fuzz-duration_from_float	0.28% <ø> (ø)
fuzz-encrypted_client_parsing	7.59% <ø> (-0.15%)	⬇️
fuzz-encrypted_server_parsing	10.13% <ø> (-0.57%)	⬇️
fuzz-ipfilter	2.59% <ø> (ø)
fuzz-key_exchange_request_parsing	4.99% <ø> (-1.04%)	⬇️
fuzz-key_exchange_response_parsing	3.80% <ø> (-0.04%)	⬇️
fuzz-packet_keyset	5.86% <ø> (-0.10%)	⬇️
fuzz-packet_parsing_sound	6.32% <ø> (-1.99%)	⬇️
fuzz-record_encode_decode	4.68% <ø> (ø)
test-aarch64-apple-darwin	83.18% <84.00%> (+<0.01%)	⬆️
test-x86_64-unknown-linux-gnu	83.09% <84.00%> (+<0.01%)	⬆️
test-x86_64-unknown-linux-musl	83.09% <84.00%> (+<0.01%)	⬆️

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