test(model): cover createAzureOpenAILanguageModel auth headers

[Mohammed Alkindi (MohammedAlkindi)]

createAzureOpenAILanguageModel sets two headers, and the comments in model.ts:214-219 say why: Authorization: Bearer <key> for managed identity, api-key for a regular key. PR #139 added the first one after 401s. typescript/tests/model.test.mjs had describe blocks for createOpenAILanguageModel, createLanguageModel and the timeout/size-limit behaviour, and nothing for Azure, so a regression to the single-header shape would not have been caught.

This adds a createAzureOpenAILanguageModel block asserting both headers, following the existing setupFetch / capturedRequests pattern in the file.

Verified it actually catches the regression by deleting the Authorization line from the built Azure factory only, leaving the OpenAI one intact, and re-running:

without the header   1 failed, 31 passed
                     actual: undefined
                     expected: 'Bearer azure-test-key'

with the header      32 passed

Could not verify: npm ci from a clean node_modules runs the workspace prepare lifecycle, which does not complete on this machine, so I installed with --ignore-scripts and built with npx tsc -p src before running node --test. Same compiled output, different entry point.