Fix crashes from unguarded str()/repr() on invalid attribute keys/values

[piyush182004]

Description

Two call sites use "best-effort stringify" as a fallback for attribute
keys/values and log exception messages, but neither guards the str()
call itself, so an object whose __str__/__repr__ raises crashes the
caller's application:

• opentelemetry-api/src/opentelemetry/attributes/__init__.py:
  _clean_attribute_value's docstring says an unstringifiable value "is
  replaced with None", but str(key)/str(value) were unguarded, so an
  object whose __str__/__repr__ itself raises crashed
  span.set_attribute(...) instead.

• opentelemetry-sdk/src/opentelemetry/sdk/_logs/_internal/__init__.py:
  LoggingHandler._get_attributes called str(value.args[0]) unguarded
  when translating a stdlib LogRecord's exc_info, crashing the common
  logger.exception(...) pattern whenever the raised exception's first
  arg has a raising __str__.

Both now catch the stringification failure, log a warning, and degrade
gracefully (drop the offending key, or replace the value with None / a
placeholder message) instead of propagating the exception into the
caller's application code — which observability/telemetry code should
never do.

Note: An earlier version of this PR also guarded _encode_baggage_pairs
in the W3C baggage propagator. That change has been dropped per review
(baggage keys and values are required to be strictly UTF-8 strings, so
W3CBaggagePropagator is not the place to guard against a raising
__str__/__repr__). The branch has been rebased onto the latest main
and squashed to a single commit.

Fixes #5597

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Added 4 regression tests across the two affected test files:

• test_clean_attribute_value_raising_repr_does_not_crash,
  test_bounded_attributes_raising_repr_value_does_not_crash,
  test_bounded_attributes_raising_repr_key_does_not_crash
  (opentelemetry-api/tests/attributes/test_attributes.py)
• test_log_record_exception_with_raising_arg_does_not_crash
  (opentelemetry-sdk/tests/logs/test_handler.py)

Verified each test is load-bearing: reverting the matching production fix
makes the test fail with the original crash traceback, and restoring the
fix makes it pass.

Ran the affected opentelemetry-api and opentelemetry-sdk suites, plus
ruff check, ruff format --check, and pylint (rated 10.00/10) — all
clean.

• Test A: test_clean_attribute_value_raising_repr_does_not_crash

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

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[piyush182004]

Still active on this, just waiting on review. Happy to rebase or split it up if that helps.

[piyush182004]

@herin049 can you please review this once?