Allow greedy decoding with 0.0 temperature

[joerunde]

Description

This PR adds the optional tag to the temperature field so that we can test presence in the server, and use greedy decoding if a user passes 0.0 temperature. This is to reduce confusion with the OpenAI api, which doesn't have a sampling/greedy flag and just uses zero temperature to enable greedy decoding.

As is, users will see an error about the temperature being too low if they pass in a temperature in the range (0, 0.05), but if they send in exactly 0, we instead default back to 1. This causes users to open bug reports about greedy mode returning random results. This PR also removes the low temperature check, since I think it would be confusing to disallow low temperatures but explicitly allow zero.

How Has This Been Tested?

I ran a server with vllm-tgis-adapter@main installed, and booted up one grpcui instance so that it would pick up the old protobuf definition. I then stopped the server, installed vllm-tgis-adapter@zero-temperature-support, booted the server again, and started a second grpcui instance so it would pick up the new protobuf definition. I verified that both:

• Unset temperature and temperature==0 requests with the old protobuf defs returned random results
• Unset temperature requests with the new protobuf defs returned random results
• Temperatue==0 requests with the new protobuf defs returned greedy results

Merge criteria:

• The commits are squashed in a cohesive manner and have meaningful messages.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 62.53%. Comparing base (2c80c72) to head (1b2f097).

Files with missing lines	Patch %	Lines
src/vllm_tgis_adapter/grpc/grpc_server.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #182      +/-   ##
==========================================
+ Coverage   62.46%   62.53%   +0.07%     
==========================================
  Files          28       28              
  Lines        1721     1719       -2     
  Branches      211      210       -1     
==========================================
  Hits         1075     1075              
+ Misses        544      543       -1     
+ Partials      102      101       -1     

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