[8.6] Fixed an edge case where a shard that hasn't yet received cluster topology could reject internal queries with 'Query requires unavailable slots' during the first moments after server startup - [MOD-13828]

[redisearch-backport-pull-request]

Description

Backport of #8388 to 8.6.

Describe the changes in the pull request

Fix an edge case where some shard doesn't know the topology yet, but has to reply to some internal query from another shard. We prefer the shard to accept the request rather than fail with "Query requires unavailable slots". The uninitialized shard cannot be a coordinator until it knows the topology, and this change doesn't affect that.

Main objects this PR modified

1. slots tracker

Mark if applicable

• This PR introduces API changes
• This PR introduces serialization changes

Release Notes

• This PR requires release notes
• This PR does not require release notes

If a release note is required (bug fix / new feature / enhancement), describe the user impact of this PR in the title.

• OSS users - the issue could happen in the first second of the server spawn
• RE users - should not affect production yet

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Note

Medium Risk
Changes the default slot-availability behavior at startup, which can affect whether internal queries are accepted/rejected until topology is received. While scoped to slots tracking, it touches correctness-critical routing/availability logic and relies on updated initialization assumptions.

Overview
Fixes an early-startup edge case by changing SlotsTracker initialization to treat a shard with no known cluster topology as owning the full slot range (0..16383), avoiding transient "unavailable slots" rejections.

This removes the const fn/derived Default path (adds a manual Default impl) and updates unit tests to explicitly simulate import/no-local-slots scenarios via set_local_slots(&[]). The FFI thread-local tracker initialization is adjusted accordingly to use the non-const constructor.

^{Written by Cursor Bugbot for commit 00aa595. This will update automatically on new commits. Configure here.}

[jit-ci]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 85.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.83%. Comparing base (ab12b48) to head (00aa595).
⚠️ Report is 2 commits behind head on 8.6.

Files with missing lines	Patch %	Lines
...c/redisearch_rs/slots_tracker/src/slots_tracker.rs	85.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              8.6    #8395   +/-   ##
=======================================
  Coverage   83.82%   83.83%           
=======================================
  Files         367      367           
  Lines       55559    55576   +17     
  Branches    14302    14319   +17     
=======================================
+ Hits        46574    46593   +19     
+ Misses       8824     8822    -2     
  Partials      161      161           

Flag	Coverage Δ
flow	84.77% <ø> (+0.01%)	⬆️
unit	50.54% <85.00%> (+0.01%)	⬆️

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