fix(openai): use versioned Mistral base URL

[arnabnandy7]

Summary

Fixes #1937 by updating the Mistral OpenAI-compatible base URL from:

https://api.mistral.ai

to:

https://api.mistral.ai/v1

The OpenAI Java client appends /chat/completions, so Mistral requests now target the correct endpoint:

https://api.mistral.ai/v1/chat/completions

The change is provider-specific; generic and other named provider URLs remain unchanged.

Testing

• Added regression coverage for the Mistral factory’s exact base URL.
• Ran the focused factory and validation tests.
• 12 tests passed with no failures or errors.
• git diff --check passed.

Test disclaimer

The live Mistral BYOK integration test was not run because MISTRAL_API_KEY was unavailable. It should be run with a valid credential before merging:

MISTRAL_API_KEY=<key> mvn -pl embabel-agent-openai \
  -Dtest=OpenAiCompatibleModelFactoryByokIT \
  test

[arnabnandy7]

@azanux need your assistance here to test this due to missing Mistral key. TIA

[azanux]

@arnabnandy7 will check in the evening

[igordayen]

@arnabnandy7 - looks good to me

[arnabnandy7]

@arnabnandy7 - looks good to me

Thanks for the review.

[azanux]

@arnabnandy7 , All good for me.

Ran the IT test with mistral , working

[arnabnandy7]

@arnabnandy7 , All good for me.

Ran the IT test with mistral , working

Thanks 😊👍

[igordayen]

@arnabnandy7 - could you please rebase, thank you

[arnabnandy7]

@arnabnandy7 - could you please rebase, thank you

Will do it by EOD, IST hours.

[arnabnandy7]

@igordayen conflicts resolved, sorry for the delay