shellenv: emit zsh fpath in nested/child shells

[sumanthratna]

The early-return idempotency check skipped all output when PATH already contained Homebrew's bin/sbin. Unlike PATH, zsh's fpath is a shell-local array not inherited by child processes, so completions broke in tmux/zellij panes and nested zsh shells.

Move shell detection before the early return and always emit the fpath line for zsh.

Fixes #21879

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• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests (excluding integration tests) for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?
  • yes; brew lgtm complains about unrelated typechecking config

==> brew style --changed --fix
Library/Homebrew/test/cmd/shellenv_spec.rb:1:1: C: Sorbet/StrictSigil: Sorbet sigil should be at least strict got false.
# typed: false
^^^^^^^^^^^^^^
Library/Homebrew/test/cmd/shellenv_spec.rb:1:1: C: Sorbet/TrueSigil: Sorbet sigil should be at least true got false.
# typed: false
^^^^^^^^^^^^^^

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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.
  • Claude Code (Opus 4.6 (1M context) with high effort) was used to read the issue, explore the relevant source code (shellenv.sh, shellenv_spec.rb, bin/brew), design the fix, and implement the changes.
  • The changes were manually verified by:
    1. Smoke-testing brew shellenv zsh with PATH set to trigger the early return — confirmed it now emits the fpath[1,0]= line (previously produced zero output)
    2. Smoke-testing brew shellenv bash with the same PATH — confirmed it still produces no output on early return (no regression)
    3. Running brew style, brew typecheck, and brew tests --only=cmd/shellenv — all pass

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[Copilot]

Pull request overview

Fixes brew shellenv for zsh in nested/child shells by ensuring the zsh fpath initialization is still emitted even when the PATH idempotency/early-return condition is met (so zsh completions work in tmux/zellij panes and nested zsh sessions).

Changes:

• Move shell detection ahead of the PATH-based early return in shellenv.sh.
• When early return triggers, still emit the fpath[1,0]=... line for zsh.
• Add an integration spec asserting fpath output for brew shellenv zsh even when PATH already contains the Homebrew prefix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/cmd/shellenv.sh	Reorders early-return logic and ensures zsh fpath is emitted in the idempotent PATH case.
Library/Homebrew/test/cmd/shellenv_spec.rb	Adds an integration test covering the zsh fpath output when PATH already includes Homebrew.

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