feat(payment)!: create_mppx_server consumes dict[str, *RailSpec]

[vvillait88]

Summary

Drop the legacy MppxRails / TempoChargeRail / TempoSessionRail / StripeRail config wrappers; create_mppx_server now consumes the canonical *RailSpec types every other helper (build_accepted_methods, build_how_to_pay, mpp_payment_handler, ...) already takes.

Migration

# before
mpp = await create_mppx_server(
    secret_key=...,
    rails=MppxRails(tempo=TempoChargeRail(recipient=...)),
)

# after
mpp = await create_mppx_server(
    secret_key=...,
    rails={"tempo": TempoRailSpec(recipient=...)},
)

Keys are rail names (tempo, tempo_session, stripe); values are the matching *RailSpec. First resolvable rail in dict-insertion order wins.

Test plan

• pytest green (1064 tests pass; coverage 95.24%)
• ty check clean
• ruff check clean
• Example doc updated