fix(auth): synthesize AuthInfo(kind="bearer") in _build_request_context

[bokelley]

Closes #576

Summary

Completes the bearer-adopter typed surface started by #574. PR #574 made ctx.auth_principal populate from the BearerTokenAuthMiddleware ContextVar; this PR addresses the deeper structural gap — ctx.auth_info itself was None on bearer flows, so adopters couldn't branch on ctx.auth_info.kind to discriminate signed-request vs bearer.

Mechanic

_build_request_context now synthesizes AuthInfo(kind="bearer", principal=<from current_principal>, credential=None) when no AuthInfo is supplied and the bearer ContextVar is populated.

credential=None is passed explicitly to bypass the AuthInfo.__post_init__ flat-field synthesis path and its accompanying DeprecationWarning — see context.py:396-426: the sentinel default fires synthesis, an explicit None does not. This makes the synthesis future-proof past the 4.5.0 synthesis removal.

The synthesized AuthInfo carries only kind + principal — bearer tokens are opaque to the SDK, so we don't know key_id / scopes. Adopters who want richer data can write their own context_factory.

Behavior shift

Bearer adopters: ctx.auth_info: None → AuthInfo(kind="bearer", ...). Adopter code that gated on if ctx.auth_info is None to detect bearer flow now needs ctx.auth_info.kind == "bearer" instead. Mirrors the #574 callout style.

Test plan

• Extended test_build_request_context_falls_back_to_bearer_context_var to assert ctx.auth_info.kind == "bearer" and ctx.auth_info.principal == bearer_principal
• Added test_build_request_context_bearer_auth_info_does_not_warn pinning the no-DeprecationWarning behavior (the spike's "(b) is clean" claim)
• test_build_request_context_auth_info_takes_precedence_over_bearer_var still passes — explicit AuthInfo wins over the bearer fallback
• Full suite: pytest tests/ — 4168 passed, 24 skipped, 1 xfailed
• ruff check src/, mypy src/adcp/ — clean