Support refreshing the BYOK provider credential without restarting the CLI

### Describe the feature or problem you'd like to solve

When using a BYOK provider with a short-lived bearer credential (e.g. an Entra ID / Azure AD OAuth access token, an AWS STS token, or any OIDC JWT), the Copilot CLI reads COPILOT_PROVIDER_API_KEY (and any custom headers) once at process startup and reuses that static value for the lifetime of the interactive session.

Entra access tokens live ~60–90 minutes. In a long interactive coding session the token expires mid-conversation, the upstream gateway (e.g. Azure API Management running validate-jwt) starts returning HTTP 401, and the CLI has no way to obtain a fresh credential short of killing and relaunching the process — which loses the conversation context.

This forces BYOK deployments that want true per-user identity (rather than a long-lived static API key) to run an external local reverse-proxy sidecar purely to re-mint and inject a fresh token on each request. That sidecar is the only reason JWT-based auth can't be the simple default for enterprise/regulated (e.g. Azure Government) BYOK setups.

Note: this is not solved by custom-header support (#3399). Custom headers would still be read once at startup and held static. The gap is specifically the refresh of a short-lived credential during a live session.

### Proposed solution

Provide a way for the CLI to obtain a fresh credential per request (or on 401), e.g. one of:

Credential command (preferred). A new env var, e.g. COPILOT_PROVIDER_API_KEY_COMMAND="az account get-access-token --scope /.default --query accessToken -o tsv". The CLI executes it to obtain the credential, caches the result, and re-executes it when the credential is near expiry or when the provider returns 401. Mirrors the well-established credential_process / credHelpers patterns in the AWS CLI, kubectl exec-credential, Git credential helpers, and Docker credential helpers.

File-backed credential. COPILOT_PROVIDER_API_KEY_FILE=/path/to/token where the CLI re-reads the file per request (an external az/cron/sidecar keeps it fresh). Simpler but weaker than option 1.

Native OAuth client-credential / refresh-token flow for COPILOT_PROVIDER_TYPE=azure that the CLI manages internally (mint + silent refresh).

Reactive trigger: on a 401 from the provider, the CLI should refresh the credential once and retry the request transparently before surfacing the error.

### Example prompts or workflows

export COPILOT_PROVIDER_TYPE=azure
export COPILOT_PROVIDER_BASE_URL=https://my-apim.azure-api.us/openai
export COPILOT_MODEL=gpt-5.1
### CLI runs this to get a token, and re-runs it automatically before expiry / on 401:
export COPILOT_PROVIDER_API_KEY_COMMAND='az account get-access-token --scope 5c211941-.../.default --query accessToken -o tsv'
copilot # stays authenticated across a multi-hour session, no restarts

### Additional context

Validated working today (2026-06-03) against an Azure APIM gateway: an Entra JWT in the api-key header passes validate-jwt and returns a gpt-5.1 completion (HTTP 200); a bad token returns HTTP 401. The only operational gap for interactive use is that the token cannot be refreshed in-session.

Prior art for "run a command to get a credential": AWS CLI credential_process, kubernetes client-go exec credential plugins, Git/Docker credential helpers.

Relationship to other issues:
#3399 (custom headers): complementary cleanup; would let the JWT ride in a real Authorization header but does not address refresh.
#3448 (extra request params): unrelated (request body params, not auth).