feat(serialization): implement truncation logic for rendered values

[imrichardwu]

This pull request introduces a shared helper for truncating long rendered template fields, so truncation is handled consistently everywhere. The goal is to ensure users always get a clear truncation message and, when there is enough space, see the truncated content wrapped in quotes. The new logic is applied across both the Airflow core and the task SDK, and is backed by thorough unit tests.

Truncation logic changes

• Added a new _truncate_rendered_value helper in both airflow/serialization/helpers.py and airflow/sdk/execution_time/task_runner.py to centralize truncation behavior. The helper prioritizes a clear truncation message and preserves quoted content when possible.
• Updated existing serialization code to use _safe_truncate_rendered_value instead of the previous inline truncation logic, removing duplication and keeping behavior consistent between the two codebases.

Tests

• Added a new test_helpers.py unit test module to validate the truncation helper.
• The tests cover edge cases such as very small max_length values, handling of quoted strings, and exact expected output for truncated values.

related: #59877

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
• Used ChatGPT to help create and edit my test cases that were mentioned by a reviewer in a closed pr.

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.

[SameerMesiah97]

I think this needs some refactoring. The current implementation looks functional but it is not very clear. I think the algorithm should be as follows:

1. If max_length <= 0, return ""

2. Build the truncation message once and, if max_length is smaller than its length, return it immediately.

3. Determine the quoting strategy and compute the formatting overhead (prefix + quotes + suffix) and calculate available space.

4. If available space is less than MIN_CONTENT_LENGTH, return the truncation message only.

5. Otherwise slice the content to fit and construct the final string and ensure the result does not exceed max_length, trimming it if necessary.

Currently, I think you are mixing 3, 4 and 5 when they should each be in disparate blocks of code. And this is resulting in needless duplication.

Also, I was just wondering why this has to be duplicated across 2 files? Perhaps, a maintainer/committer could weigh in on this but is it possible to have this in airflow.utils.helpers instead?