Python: fix(core): point @experimental warnings at user code, not stdlib internals

[eavanvalkenburg]

Motivation and Context

When an @experimental ABC is subclassed (e.g. MemoryStore, SkillResource), the emitted ExperimentalWarning was attributed to internal stdlib frames instead of the user's code. On modern CPython this surfaced as:

/.../python3.13/abc.py:106: ExperimentalWarning: [HARNESS] MemoryStore is experimental ...
  cls = super().__new__(mcls, name, bases, namespace, **kwargs)

The cause: the wrappers installed by @experimental called warnings.warn with a fixed stacklevel=3, but ABCMeta inserts an extra abc.__new__ frame for ABC-driven class creation, so the warning landed inside abc.py (or <frozen abc>:106) rather than the user's class Sub(...) line.

Raised as a follow-up to a review comment on #5958.

Description

• Replace the fixed stacklevel with a frame walker (_resolve_user_frame) that starts at inspect.currentframe() and skips frames belonging to this module plus abc/functools/typing/contextlib and their submodules. The skip list is matched on frame.f_globals['__name__'] rather than __file__, because frozen stdlib modules report <frozen abc> as the filename.
• Emit via warnings.warn_explicit at the resolved (filename, lineno, module) so recorded warnings point at user code; fall back to warnings.warn(stacklevel=2) only if no user frame is found.
• Install a single-line warnings.formatwarning specifically for FeatureStageWarning so the output is file:line: ExperimentalWarning: [FEATURE_ID] Name is experimental ... with no secondary source-snippet line (the class/function name is already in the message). Other warning categories delegate to the stdlib default formatter unchanged.
• Added a regression test that subclasses an @experimental ABC inside warnings.catch_warnings and asserts the recorded filename equals the test file.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[moonbox3]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_feature_stage.py	163	10	93%	104, 150, 205, 216, 236, 274, 285, 291, 322, 350
TOTAL	35332	4108	88%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
7011	30 💤	0 ❌	0 🔥	1m 52s ⏱️

[Copilot]

Pull request overview

Adjusts Python feature-stage warning emission so @experimental warnings are attributed to the user’s callsite (not stdlib/internal frames), including the ABC subclassing case that previously surfaced as <frozen abc>:....

Changes:

• Replaces fixed stacklevel warning emission with a stack frame walker that resolves a “user” frame and emits via warnings.warn_explicit.
• Installs a custom warnings.formatwarning implementation to render FeatureStageWarning categories as single-line warnings.
• Adds a regression test covering subclassing an @experimental ABC and asserting the warning filename points at the test/user file.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
python/packages/core/agent_framework/_feature_stage.py	Resolves user frames for warnings, switches to warn_explicit, and installs a single-line formatter for feature-stage warnings.
python/packages/core/tests/core/test_feature_stage.py	Adds regression coverage for ABC subclass warning attribution.