[BUG] prevent integer overflow in HNSW index resize

[rescrv]

Description of changes

Replace unchecked arithmetic (capacity * 2, len + additional) with a
central required_hnsw_capacity function that uses checked_add,
checked_mul, and checked_next_power_of_two, returning a typed
HnswCapacityError::Overflow on overflow instead of silently wrapping.

All six callsites in fast_writer, spann/types, distributed_hnsw, and
local_hnsw now route through this function, which encodes two growth
strategies (Double and NextPowerOfTwo) and an optional resize-at-
capacity flag to preserve each callsite's existing semantics.

Also constrain resize_factor to [1.0, 5.0] on the Rust
HnswIndexConfig via a validate(range) annotation, with tests for
both boundary rejection and overflow at usize::MAX.

Test plan

CI

Migration plan

N/A

Observability plan

N/A

Documentation Changes

N/A

Co-authored-by: AI

[github-actions]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

[blacksmith-sh]

Found 1 test failure on Blacksmith runners:

Failure

Test	View Logs
chroma-types/ collection_schema::tests::proptests::reconcile_schema_and_config_matches_schema_only_pa th	View Logs

[rescrv]

In favor of #7098