Move sampler argument logic to respective samplers

[rads-1996]

Description

Fixes - #32499

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

This PR moves OTEL_TRACES_SAMPLER_ARG parsing/defaulting logic into the sampler implementations themselves (ApplicationInsightsSampler and RateLimitedSampler), aligning behavior with the referenced issue and reducing reliance on higher-level configuration layers.

Changes:

• Updated ApplicationInsightsSampler to optionally read OTEL_TRACES_SAMPLER_ARG when no explicit ratio is provided, and to default (log) on invalid inputs rather than raising.
• Updated RateLimitedSampler to optionally read OTEL_TRACES_SAMPLER_ARG when no explicit traces-per-second value is provided, and to default (log) on invalid inputs rather than raising.
• Expanded unit tests to cover env-var and invalid-input behaviors for both samplers.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/trace/test_sampling.py	Updates tests for ApplicationInsightsSampler’s new env-var/defaulting behavior.
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/trace/test_rate_limited_sampling.py	Adds tests for RateLimitedSampler’s new env-var/defaulting behavior and invalid inputs.
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/export/trace/_sampling.py	Implements env-var parsing and defaulting within ApplicationInsightsSampler.
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/export/trace/_rate_limited_sampling.py	Implements env-var parsing and defaulting within RateLimitedSampler.