fix: avoid hash collision in caveat context

[tstirrat15]

Description

Someone pointed out to us that if you use a nested list of lists in a caveat context, it was possible under our current serialization and hashing logic to have a serialization collision and therefore a dispatch cache hash collision.

This fixes that by using deterministic proto marshalling of the caveat context Struct object.

Changes

• Update context_hash.go to use deterministic marshalling
• Propagate the error signature change through the stack
• change hashableContext to hashableContextString to satisfy the interface

Testing

Review

[codecov]

Codecov Report

❌ Patch coverage is 62.22222% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.01%. Comparing base (312216f) to head (23b7171).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/dispatch/keys/computed.go	57.15%	4 Missing and 2 partials ⚠️
internal/dispatch/keys/keys.go	50.00%	4 Missing and 2 partials ⚠️
internal/services/v1/hash.go	40.00%	2 Missing and 1 partial ⚠️
pkg/caveats/context_hash.go	83.34%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3065      +/-   ##
==========================================
- Coverage   76.05%   76.01%   -0.03%     
==========================================
  Files         486      486              
  Lines       59456    59440      -16     
==========================================
- Hits        45211    45178      -33     
- Misses      10972    10981       +9     
- Partials     3273     3281       +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[miparnisari]

why do we need an end to end test for something that could easily be encoded at a unit test level - compare the hashing of {"x": ["a", [], "b"]} and {"x": [["a"], "b"]} and ensures they are not the same?

the test you wrote, if it fails, tells us very little about where the problem could be.