refactor: clean up structural dispatch patterns

[WH-2099]

Summary

• Convert fixed dispatch and string parsing cleanup candidates from issue refactor: track structural cleanup candidates from review follow-up #329 to clearer standard patterns.
• Replace duplicated prompt/tool conversion logic with shared helpers.
• Normalize OpenAI-compatible endpoint URL joining and GitHub branch ref parsing.

Validation

• just check
• just test

Closes #329

[gemini-code-assist]

Code Review

This pull request introduces several refactorings and cleanups across the codebase, including simplifying enum value lookups using direct instantiation, utilizing modern string methods like removeprefix and removesuffix, leveraging typing.get_args and typing.get_origin in the documentation generator, and extracting common Notion parsing logic. It also introduces a centralized ensure_prompt_message helper to instantiate role-specific prompt messages. The review feedback highlights two key areas for improvement: first, copying parameter.input_schema in strategy.py to prevent unintended side-effects from mutating a shared dictionary; second, adding defensive type checking in ensure_prompt_message to gracefully handle non-mapping inputs instead of raising an AttributeError.

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