[Config] Unify retry environment variable naming: AZURE_PS_HTTP_MAX_RETRIES vs PS_HTTP_MAX_RETRIES

## Problem

Retry configuration uses two different environment variable names:

- `PS_HTTP_MAX_RETRIES` (Track 1)
- `AZURE_PS_HTTP_MAX_RETRIES` (Track 2)

This inconsistency causes confusion and unclear precedence for users.

## Goal

Standardize retry configuration to a single, clear environment variable.

## Proposal

- Use `AZURE_PS_HTTP_MAX_RETRIES` as the canonical variable
- Continue supporting `PS_HTTP_MAX_RETRIES` as an alias (non-breaking)
- Define precedence if both are set (canonical wins)
- Document behavior and migration guidance

## Acceptance Criteria

- [ ] Single canonical env var defined
- [ ] Legacy var supported or deprecated clearly
- [ ] Precedence rules implemented
- [ ] Docs updated

No files or tests are named. Search the repository for PS_HTTP_MAX_RETRIES and AZURE_PS_HTTP_MAX_RETRIES, then trace their configuration readers and related tests or documentation; done means the canonical variable, legacy alias, precedence, migration guidance, and acceptance criteria are covered.

Written by the indexing model from the issue text.