refactor(cli): replace esbuild with tsdown

[overbalance]

What problem is this solving?

Aligns CLI build tooling with the main SDK by using tsdown (rolldown-based), reducing build tool fragmentation.

Short description of changes

• Replace esbuild with tsdown for CLI compilation (tsc runs first for type checking)
• Convert validation script from bash to TypeScript for cross-platform support (to match SDK)
• Remove custom validation checks now handled by tsdown (publint, shebang)
• Delete unused scripts/measureSize.ts
• Output changed from dist/index.js to dist/index.mjs (default extension from tsdown)

Testing

• npm run cli:compile produces valid output
• npm run cli:validate passes (es-check + integration test)
• npm run cli:test passes

[overbalance]

license and readme are always included by npm

[github-actions]

Platform Tests (vite-7 gzip: 47.99KB)

vite-7 Platform Tests

	Total Uncompressed Size	Total Gzip Size
vite-7 - esnext	+164.67 KB	+47.99 KB
vite-7 - es2015	+171.51 KB	+49.67 KB

vite-otel-latest Platform Tests

	Total Uncompressed Size	Total Gzip Size
vite-otel-latest - esnext	+163.74 KB	+49.69 KB
vite-otel-latest - es2015	+170.50 KB	+51.32 KB

webpack-5 Platform Tests

	Total Uncompressed Size	Total Gzip Size
webpack-5 - esnext	+121.11 KB	+43.18 KB
webpack-5 - es2015	+122.22 KB	+44.65 KB

Chrome DevTools Protocol Tracing (Script: 102.83ms, Heap: 12.87MB)

	Number of Requests	Size of Requests	Script Duration	Task Duration	Heap Used Size
Requests	+3 requests	+29.72 KB
Page Loaded			+13.19 ms	+6.89 ms	+0.91 MB
Generate 100 fetch requests			+15.92 ms	+64.56 ms	+1.71 MB
Generate 100 XHR requests			+40.24 ms	+115.13 ms	+2.31 MB
Click 100 buttons and generate 100 logs			+24.43 ms	+28.52 ms	+2.85 MB
Throw a 100 exceptions			+1.67 ms	+14.16 ms	+2.12 MB
End Session			+7.38 ms	+15.22 ms	+2.98 MB
Total	+3 requests	+29.72 KB	+102.83 ms	+244.49 ms	+12.87 MB

Lighthouse (Script Eval: 44.79ms)

	Difference	Description
Total Blocking Time	0 ms	Difference in Total Blocking Time: Sum of all time periods between FCP and Time to Interactive, when task length exceeded 50ms, expressed in milliseconds. Learn more about the Total Blocking Time metric.
Main Thread Time	+59.24 ms	Difference in Main Thread Time: Consider reducing the time spent parsing, compiling and executing JS. You may find delivering smaller JS payloads helps with this. Learn how to minimize main-thread work
Script Evaluation Time	+44.79 ms	Difference in Script Evaluation Time: Consider reducing the time spent parsing, compiling, and executing JS. You may find delivering smaller JS payloads helps with this. Learn how to reduce Javascript execution time.