Update READMEs for gpt-4.1 model upgrade (coordinate with docs PR)

[diberry]

Summary

PR #70 upgrades the OpenAI infrastructure from gpt-4o/gpt-4o-mini to gpt-4.1/gpt-4.1-mini with configurable params. READMEs in the sample folders still reference the old model names and need updating.

Changes needed

README updates

• ai/vector-search-agent-go/README.md — replace references to gpt-4o and gpt-4o-mini with gpt-4.1 and gpt-4.1-mini
• ai/vector-search-agent-typescript/README.md — replace references to gpt-4o and gpt-4o-mini with gpt-4.1 and gpt-4.1-mini (including .env examples)

Coordination requirement

This must ship at the same time as the corresponding quickstart/tutorial update in azure-databases-docs-pr so that the docs and samples stay in sync.

Infra changes (PR #70)

• Models: gpt-4.1-mini (chat), gpt-4.1 (synth), text-embedding-3-small (embedding)
• All use Standard tier (not GlobalStandard)
• New openAiLocation param separates OpenAI region from main resource location
• Model names, versions, and tiers are now configurable via env vars

Blocked by

• PR Update infra: gpt-4.1 models, AVM 0.10.0, tier comments #70 merge
• Corresponding docs update in azure-databases-docs-pr