fix: Use do_exchange for offline server reads to support HPA

[ntkathole]

Problem

The remote offline server uses a two-phase Arrow Flight protocol for read operations:

1. do_put — client sends command + entity data → server stores it in self.flights (an in-memory Python dict)
2. get_flight_info + do_get — client retrieves results in separate gRPC calls → server looks up self.flights

With HPA (Horizontal Pod Autoscaler) or multiple replicas, these separate gRPC calls can be load-balanced to different pods. When do_put lands on Pod A but get_flight_info hits Pod B, the server raises KeyError("Flight not found.") because Pod B's self.flights dict has no record of the flight stored on Pod A.

Client              K8s Service (round-robin)       Pod A         Pod B
  │                          │                        │             │
  ├── do_put(data) ─────────►├───────────────────────►│             │
  │                          │              flights[key] = data     │
  │                          │                        │             │
  ├── get_flight_info() ────►├─────────────────────────────────────►│
  │                          │                        │   key not in│
  │                          │                        │   flights   │
  │   ◄── KeyError("Flight not found.") ◄───────────────────────────┤

Solution

Replace the two-phase read path with Arrow Flight's do_exchange RPC, which handles both the entity data upload and result download in a single bidirectional gRPC stream. This guarantees both phases are processed by the same server pod.

Client              K8s Service           Pod A
  │                      │                  │
  ├── do_exchange() ────►├─────────────────►│
  │   (single stream)    │                  ├─ receives entity data
  │                      │                  ├─ executes query
  │   ◄── results ◄──────┤◄─────────────────┤─ streams results back

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Codecov Report

❌ Patch coverage is 63.23529% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.48%. Comparing base (fa8f06b) to head (48ad47c).

Files with missing lines	Patch %	Lines
sdk/python/feast/offline_server.py	58.00%	14 Missing and 7 partials ⚠️
sdk/python/feast/infra/offline_stores/remote.py	77.77%	3 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6846      +/-   ##
==========================================
+ Coverage   47.43%   47.48%   +0.05%     
==========================================
  Files         422      422              
  Lines       52263    52321      +58     
  Branches     7582     7590       +8     
==========================================
+ Hits        24791    24846      +55     
+ Misses      25708    25702       -6     
- Partials     1764     1773       +9     

Flag	Coverage Δ
go-feature-server	30.58% <ø> (ø)
python-unit	48.81% <63.23%> (+0.05%)	⬆️

Files with missing lines	Coverage Δ
sdk/python/feast/infra/offline_stores/remote.py	55.06% <77.77%> (+4.82%)	⬆️
sdk/python/feast/offline_server.py	35.75% <58.00%> (+6.35%)	⬆️

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[Marcus-Rosti]

I'll try testing this, but overall the logic looks right for the flight server

[shuchu]

lgtm