fix: widen generation function specs to match Context.normalize/2

[Nickcom4]

Summary

• Adds Context.prompt() type alias that matches all types accepted by Context.normalize/2
• Updates @spec on all 8 generation functions to use Context.prompt() instead of the overly narrow String.t() | list()
• Eliminates Dialyzer false-positive warnings in downstream apps that pass Context.t() structs (the recommended usage pattern)

Affected functions

All 8 specs in ReqLLM.Generation:

• generate_text/3, generate_text!/3
• stream_text/3, stream_text!/3
• generate_object/4, generate_object!/4
• stream_object/4, stream_object!/4

The Context.prompt() type

@type prompt ::
        String.t()
        | Message.t()
        | t()
        | map()
        | [String.t() | Message.t() | t() | map()]

This exactly mirrors the spec on Context.normalize/2 (lines 103-110 of context.ex).

Precedent

Same class of fix as PR #301 which widened the object_schema parameter spec for generate_object.

Test plan

• mix compile --warnings-as-errors passes (zero warnings)
• Context.prompt() type matches Context.normalize/2 spec exactly
• Using a named type alias is DRYer than repeating the union in 8 places

Closes #434