chore: Use shared validate-pr composite action

[stephanie-anderson]

Replace the inline PR validation workflow with the shared composite action
from getsentry/github-workflows#153.

This shrinks the workflow from 300+ lines to ~15 and ensures future updates
to validation logic are picked up automatically.

#skip-changelog

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

This is how your changes will appear in the changelog.
Entries from this PR are highlighted with a left border (blockquote style).

---

This PR will not appear in the changelog.

---

_{🤖 This preview updates automatically when you update the PR.}

[sentry]

Sentry Build Distribution

App Name	App ID	Version	Configuration	Install Page
SDK Size	io.sentry.tests.size	8.37.1 (1)	release	Install Build

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	306.91 ms	341.06 ms	34.15 ms
Size	0 B	0 B	0 B

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
6405ec5	310.88 ms	354.56 ms	43.69 ms
d5a29b6	298.62 ms	391.78 ms	93.16 ms
f064536	329.00 ms	395.62 ms	66.62 ms
fcec2f2	357.47 ms	447.32 ms	89.85 ms
bbc35bb	324.88 ms	425.73 ms	100.85 ms
694d587	312.37 ms	402.77 ms	90.41 ms
b8bd880	314.56 ms	336.50 ms	21.94 ms
889ecea	367.58 ms	437.52 ms	69.94 ms
e2dce0b	308.96 ms	360.10 ms	51.14 ms
83884a0	334.46 ms	400.92 ms	66.46 ms

App size

Revision	Plain	With Sentry	Diff
6405ec5	1.58 MiB	2.12 MiB	552.23 KiB
d5a29b6	1.58 MiB	2.12 MiB	549.37 KiB
f064536	1.58 MiB	2.20 MiB	633.90 KiB
fcec2f2	1.58 MiB	2.12 MiB	551.50 KiB
bbc35bb	1.58 MiB	2.12 MiB	553.01 KiB
694d587	1.58 MiB	2.19 MiB	620.06 KiB
b8bd880	1.58 MiB	2.29 MiB	722.92 KiB
889ecea	1.58 MiB	2.11 MiB	539.75 KiB
e2dce0b	0 B	0 B	0 B
83884a0	1.58 MiB	2.29 MiB	722.97 KiB