[jihtsan]

Closes #1094.

FewShotPromptTemplate validates variables from both prefix and suffix, but format left the prefix unchanged. A valid prefix: "Topic: {topic}" consequently sent the placeholder itself to the model.

Format prefix variables in one pass using the same name extraction contract as validation: trim variable names and preserve ignored brace regions as literal text. Inserted values are not rescanned, stored templates stay unchanged, and the suffix retains its existing escaping/replacement behavior. Examples, order and custom separators are preserved.

Validation (Ruby4.0.9, macOS arm64):

• Original targeted few-shot/structured-parser baseline:15 examples passed.
• On base, four new prefix regressions fail and six controls pass. Additional interim regressions caught three ignored brace forms and two validator-whitespace cases; these are fixed in the submitted implementation.
• Complete prompts spec directory: 34 examples,0failures. Covers prefix-only/shared variables, string argument keys, braces/backslashes as inserted data, literal regions, whitespace, unknown placeholders, stored values and custom separators.
• Repository-wide bundle exec standardrb, bundle exec yardoc, git diff --check: passed. YARD's existing documentation warnings remain.
• Independent Standards and Spec reviews:0findings for this head.

The upstream has no tracked Gemfile.lock; the generated validation lock and installed bundle were retained as evidence and are not committed. No LLM/provider calls were used. The complete RSpec/Postgres/vector/provider integration matrix and other Ruby/OS versions were not run.