fix(runtime): distinguish request parse errors from handler JSON errors

[jesseturner21]

Summary

• Fixes misleading "Invalid JSON in request" error when the handler (not the request) raises json.JSONDecodeError (e.g., parsing model output)
• Narrows the JSONDecodeError/UnicodeDecodeError catches to only cover request.json() parsing
• Handler-raised exceptions now correctly return HTTP 500 with the actual error message

Resolves V2192037347 — customer reported AgentCore runtime surfaces misleading error classification for downstream application JSON parsing errors.

Test plan

• Verified bug reproduction: handler JSONDecodeError previously returned 400 "Invalid JSON"
• Verified fix: handler JSONDecodeError now returns 500 with actual error
• Verified regression: actual invalid request JSON still returns 400 "Invalid JSON"
• All 311 runtime unit tests pass

[github-actions]

✅ No Breaking Changes Detected

No public API breaking changes found in this PR.

[Hweinstock]

nice fix! Is it possible to verify this with a unit test or even better an integ test?

[jesseturner21]

Note on CI

The Test (tools) failure is pre-existing and also fails on main (run 25698588721). It's caused by a KMS key policy on account 121875801285 not granting the CI role kms:Decrypt for the BrightData proxy secret. Unrelated to this change.

All relevant checks pass: Test (runtime), Test Python 3.10–3.13, Compat (runtime), Lint and Format.

[Hweinstock]

Run uv run pre-commit run --all-files
[INFO] Initializing environment for https://github.com/astral-sh/uv-pre-commit.
[INFO] Initializing environment for https://github.com/astral-sh/ruff-pre-commit.
[INFO] Initializing environment for https://github.com/pre-commit/pre-commit-hooks.
[INFO] Initializing environment for https://github.com/PyCQA/bandit.
[INFO] Installing environment for https://github.com/astral-sh/uv-pre-commit.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
[INFO] Installing environment for https://github.com/astral-sh/ruff-pre-commit.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
[INFO] Installing environment for https://github.com/pre-commit/pre-commit-hooks.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
uv-lock..................................................................Passed
ruff (legacy alias)......................................................Passed
ruff format..............................................................Failed
- hook id: ruff-format
- files were modified by this hook

1 file reformatted, 219 files left unchanged

trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check toml...............................................................Passed
check json...............................................................Passed
check yaml...............................................................Passed
check for merge conflicts................................................Passed
check for added large files..............................................Passed
debug statements (python)................................................Passed

lint failing, otherwise LGTM. Thanks for adding the test!