Regenerate C schema headers for Policy.id string field

[annacai21]

This PR regenerates the C schema headers (policy_reader.h, policy_builder.h, policy_verifier.h) via flatcc, which were stale since Policy.id changed from a UUID struct to a string in #74.

[datadog-official]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 21.66%

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[iamluc]

Are we sure this is backward compatible with existing policies already in flatbuffer?

[annacai21]

Yes. This would be a breaking wire-format change for any buffer where id was already populated. However, I checked both ways a Policy buffer gets built and confirmed that neither of them already populated the id field:

1. dd-policy-engine's converters never called PolicyAddId anywhere in this repo before PR Stamp hardcoded policies with id="hardcoded" #75
2. RC policies are compiled via dd-compile-policy from JSON, but that JSON has never contained a real id either.

Since FlatBuffers vtables mark an unset field as explicitly absent, every buffer compiled by either path so far has that slot empty, and the new reader will correctly see the field absent rather than reinterpreting old struct bytes as a string offset. This would only be unsafe if some buffer out there already had a real non-zero id, and nothing does yet.

[natitsechanski]

lgtm