feat: add tracks assignment and exposure

[zhukaihan]

Summary

Add track, or not to track, assignment and exposure events options to fetch requests.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: no

[promptless]

📝 Documentation updates detected!

New suggestion: Document FetchOptions for Python Experiment SDK
Updated existing suggestions:

• Document FetchOptions for JVM Experiment SDK
• Document exposureTrackingOption and FetchOptions for JavaScript Experiment SDK

[Copilot]

Pull Request Overview

This PR adds support for FetchOptions to control assignment and exposure tracking behavior when fetching experiment variants. The main feature allows clients to specify whether to track assignment and exposure events via custom HTTP headers.

Key changes:

• Introduced a new FetchOptions class with tracksAssignment and tracksExposure parameters
• Updated fetch, fetch_v2, and related internal methods to accept and propagate FetchOptions
• Modified the HTTP request headers to include X-Amp-Exp-Track and X-Amp-Exp-Exposure-Track based on options
• Updated GitHub Actions workflows to use actions/setup-python@v6

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

File	Description
src/amplitude_experiment/remote/fetch_options.py	New class defining options for tracking assignment and exposure events
src/amplitude_experiment/remote/client.py	Updated fetch methods to accept and propagate FetchOptions, added header logic
tests/remote/client_test.py	Added test case for FetchOptions and updated existing test assertions
.github/workflows/*.yml	Bumped actions/setup-python from v3 to v6

Comments suppressed due to low confidence (2)

src/amplitude_experiment/remote/client.py:104

• The __fetch_async_internal method doesn't pass fetch_options to __fetch_internal, causing FetchOptions to be ignored in async operations. Update line 104 to: variants = self.__fetch_internal(user, fetch_options) and add a fetch_options parameter to __fetch_async_internal.

            variants = self.__fetch_internal(user)

src/amplitude_experiment/remote/client.py:61

• The fetch_async_v2 method is missing a fetch_options parameter, creating an inconsistency with the synchronous fetch_v2 method. Add fetch_options: FetchOptions = None as a parameter and update the thread invocation on line 68 to include it.

    def fetch_async_v2(self, user: User, callback=None):

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Mixing implicit and explicit returns may indicate an error, as implicit returns always return None.