feat: Browserbase proxy, stealth, and image blocking via env vars

[JustYannicc]

Summary

Adds environment variable support for configuring Browserbase sessions with proxy, advanced stealth, and image blocking. Also includes the fix for #912 (415 error on session creation).

New environment variables:

Variable	Description	Example
BROWSERBASE_PROXY	Enable residential proxy	1
BROWSERBASE_ADVANCED_STEALTH	Enable advanced stealth (custom Chromium)	1
BROWSERBASE_OS	OS fingerprint for stealth	windows, mac, linux, mobile, tablet
BROWSERBASE_BLOCK_IMAGES	Block image loading via CDP Fetch interception	1

Implementation:

• providers.rs: Builds the session creation body from env vars (proxies, browserSettings with advancedStealth/os)
• actions.rs: After CDP connection, registers abort routes for image extensions and enables Fetch interception when BROWSERBASE_BLOCK_IMAGES=1

Note on image blocking: Per Browserbase docs, blocking images via request interception is not recommended on sites with anti-bot protection, as image/font loading is used to verify legitimate browsers. This is best suited for non-visual automation on sites without such protections.

Test plan

• Built locally (cargo build --release)
• Verified 415 fix: -p browserbase creates sessions successfully
• Verified proxy: BROWSERBASE_PROXY=1 routes through residential IP (confirmed different IP via httpbin.org/ip)
• Verified image blocking: BROWSERBASE_BLOCK_IMAGES=1 registers Fetch abort routes for image extensions
• Confirmed image blocking + anti-bot sites conflict (as documented by Browserbase)

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[vercel]

Suggested change

	"**/*.png", "**/*.jpg", "**/*.jpeg", "**/*.gif",
	"**/*.webp", "**/*.svg", "**/*.ico", "**/*.avif",
	"*.png", "*.jpg", "*.jpeg", "*.gif",
	"*.webp", "*.svg", "*.ico", "*.avif",

Image blocking patterns like **/*.png never match any URL in the route matching logic, so images are intercepted by CDP but then continued instead of aborted, adding latency without blocking anything.

[ctate]

Thanks for the contribution, @JustYannicc! Clean implementation — the env var approach is consistent and the 415 fix is a welcome bug fix.

A few suggestions before merging:

1. BROWSERBASE_OS validation — Currently any string is accepted and passed to the API. Consider validating against the documented values (windows, mac, linux, mobile, tablet) and returning a clear error for invalid input, rather than letting the API reject it.

2. Silent Fetch.enable failure — The let _ = on the Fetch.enable CDP call silently swallows errors. If this fails, image blocking won't work but the user won't know. A warning log would help debuggability.

3. Image blocking patterns — The glob-style patterns (**/*.png) only match by file extension and will miss images served without extensions (e.g., CDN URLs like https://cdn.example.com/img/abc123). Worth noting in docs or comments that this is extension-based filtering only.

Otherwise LGTM — nice docs in the changeset and good call noting the anti-bot caveat.