test(pydantic_ai): Fix instructions test for pydantic-ai >=2.36.0

[ericapisani]

Build the agent with public instructions/system_prompt kwargs instead of poking internal _instructions/_system_prompts attributes, and account for the joiner between multiple instructions changing from a single newline to a blank line in pydantic-ai 2.36.0.

There's normalization of instructions occurring under the hood as of 2.36.0 that we don't have a reason to interfere with within the tests, so moving the configuration of instructions and the system prompt to using the public Agent API.

Fixes PY-2755
Fixes #7306

[linear-code]

PY-2755

[github-actions]

Codecov Results 📊

✅ 121930 passed | ⏭️ 6762 skipped | Total: 128692 | Pass Rate: 94.75% | Execution Time: 437m 25s

📊 Comparison with Base Branch

Metric	Change
Total Tests	—
Passed Tests	—
Failed Tests	—
Skipped Tests	—

✨ No test changes detected

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2482 uncovered lines.
✅ Project coverage is 90.31%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    90.28%    90.31%    +0.03%
==========================================
  Files          193       193         —
  Lines        25621     25621         —
  Branches      9438      9438         —
==========================================
+ Hits         23132     23139        +7
- Misses        2489      2482        -7
- Partials      1436      1436         —

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