feat: make builders monadic via TelegramEx.Effect

[lsdrfrx]

Description

Adds the new TelegramEx.Effect execution model and adapts builders to use it.

Builders now accept either a raw handler context map or an existing TelegramEx.Effect, and return an effect. This keeps the existing pipeline style while allowing builder chains to carry errors instead of raising or
returning early. For example, file-based builders can now preserve file read failures in the effect, and API request failures are also stored in the same flow.

The server no longer needs to know effect internals directly. TelegramEx.Effect.to_result/1 converts an effect back into a normal handler result, and TelegramEx.Server continues to handle FSM transitions, :ok, :pass,
and errors as regular handler outcomes.

A shared TelegramEx.Builder.put_payload/3 helper was added to avoid duplicating payload mutation logic across builders.

How it works now:

• Builders call Effect.wrap/1, so users can start pipelines with the usual ctx.
• Payload-only steps use Effect.map_ctx/2.
• Fallible steps use Effect.then/2.
• File read failures are stored as {:file, reason}.
• API failures are stored as effect errors.
• Server.handle_result/4 calls Effect.to_result/1 and then processes the normalized result.
• Common payload updates are centralized in TelegramEx.Builder.put_payload/3.

This keeps the public DX close to the old style:

  ctx
  |> Document.path("/tmp/report.pdf")
  |> Document.caption("Report")
  |> Document.send(chat_id)

but the pipeline can now safely carry errors through the builder chain.

Ready Checklist

• Implementation: code works, types are specified, dialyzer passes
• Documentation: functions have @doc, README updated if needed
• Tests: tests added/updated, mix test passes locally

Related Issues

Closes #21, #20