[FC-0118] docs: Add ADR for ensuring GET requests are idempotent

[taimoor-ahmed-1]

Description

This PR adds ADR-0030 to document and standardize idempotent GET behavior across Open edX REST APIs. The ADR establishes that GET must be read-only and that any state mutations or tracking writes should be handled via explicit write endpoints or decoupled async telemetry.

What changed
Added edx-platform/docs/decisions/0030-ensure-get-requests-are-idempotent.rst
Context on current non-idempotent GET risks
Decision and required implementation rules
edx-platform-specific relevance notes
Code examples showing anti-pattern and preferred approach
related issue: #38248

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[Abdul-Muqadim-Arbisoft]

Could we remove this line to maintain consistency across similar ADRs?

[feanil]

@bmtcril I'd love to get your thoughts on this. @taimoor-ahmed-1 while I'm not generally opposed to idempotent GET requests, I think an exception for telemetry data should exist. Pushing that to async calls increases cost of what should be low overhead calls.

[bmtcril]

I think there is a difference between GETs that side-effect the domain or transactional state and those that trigger changes in a different domain. Analytics and other telemetry are separate domains from, say, student state and aren't modifying any data relevant to the GET request, which is what the distinction is really about.

I totally agree with things like the student state not being modified by side-effects in GET requests, but I think logging and telemetry are separate concerns.