[codex] add input-field jinja rendering support

[darinkishore]

Summary

This change adds first-class #[render(jinja = "...")] support for #[input] fields in Signature-derived structs and wires runtime rendering in ChatAdapter.

Problem

Input rendering previously supported #[format(...)] serialization only. There was no typed, per-field templating path for inputs, and no compile-time diagnostics for invalid #[render] usage.

Root Cause

FieldMetadataSpec only carried format hints and the derive macro only parsed #[format]/alias/constraints. The adapter formatting path had no metadata-driven render mode for input fields.

What Changed

• Added InputRenderSpec in schema metadata and propagated it through FieldSchema.
• Extended #[derive(Signature)] to parse and validate #[render(jinja = "...")]:
  • compile-time Jinja syntax validation
  • conflicts with #[format]
  • restricted to #[input]
  • duplicate/invalid-arg diagnostics
  • alias collision diagnostics after alias resolution
• Updated ChatAdapter to render input fields based on metadata:
  • default behavior (raw string or JSON)
  • explicit #[format(...)]
  • Jinja rendering with context (this, input, field, vars)
• Added adapter Jinja environment setup aligned with existing BAML behavior and explicit helper filters (regex_match, sum, truncate) while keeping dependency declarations unchanged.
• Added/updated macro tests and UI tests for new diagnostics.
• Updated docs for #[render] usage and runtime behavior.

Validation

• cargo test -p dspy-rs --test test_input_format -q
• cargo test -p dsrs_macros -q

Notes

Dependency source for minijinja remains unchanged (boundaryml/minijinja on current branch pinning strategy).