test: add hook ordering regression coverage

[tatakaisun]

Pull Request

Issue

Fixes #1000

Description

Add regression coverage for hook call-site ordering and payload mutation before downstream processing.

This PR also applies session_pre_init model_id mutations when constructing the backend, so the hook-mutated model_id_str is passed to the backend constructor.

Summary

• Adds regression tests for hook call-site ordering and payload mutation handling.
• Tests that generation_pre_call mutations are applied before backend generation.
• Tests that session_pre_init mutations are applied before backend construction.
• Updates start_session() to pass the hook-mutated model_id_str to the backend constructor.

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code was added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

Manual checks:

• git diff --check passed.
• Targeted pytest was not run locally because uv is not available on PATH.

Attribution

• AI coding assistants used

Assisted-by: OpenAI Codex

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[psschwei]

Thanks for the PR @tatakaisun !
Quick note: we recently added the DCO bot to this repo, and as an unfortunate consequence any PR opened before that gets blocked by it.
To kick the bot and get it to run, could you send a new commit:

git commit --amend --no-edit
git push --force-with-lease

[tatakaisun]

Thanks for the heads up! I’ll amend the commit and push an updated revision to retrigger the DCO bot.

[psschwei]

looks like you're failing the linter

[tatakaisun]

Thanks for the heads-up. I pushed a formatter-only update for the failing lint check. No logic changes were made.

[psschwei]

LGTM
Thanks @tatakaisun !