chore: Add melos commands for example app and e2e tests

[hkarmoush]

📜 Description

Adds two melos scripts:

• melos run example:flutter -- <platform> — builds and runs the Flutter example app (delegates to the existing packages/flutter/example/run.sh, so there's still one source of truth for the build logic).
• melos run e2e:dart — runs the Dart e2e suite against sentry.io (mirrors .github/workflows/e2e_dart.yml, requires SENTRY_AUTH_TOKEN_E2E locally).

Also documents the existing and new scripts in a "Common commands" table in CONTRIBUTING.md, and fixes a small bug in run.sh where calling it with no arguments crashed on an unbound variable (set -u) instead of printing the usage message.

💡 Motivation and Context

Closes #3660

Building the example app and running the e2e suite both require remembering raw shell commands that live in a few different places (run.sh, CI workflow files). This wraps them behind melos run, which is discoverable via melos run --list.

I kept this scoped to the two examples the issue calls out rather than guessing at everything "and so on" might cover.

💚 How did you test it?

• melos run example:flutter -- "" to confirm the script forwards args correctly and the run.sh usage guard fires as expected.
• melos run e2e:dart to confirm it resolves dependencies and fails fast with a clear error when SENTRY_AUTH_TOKEN_E2E isn't set (same behavior as CI without the secret).
• bash -n packages/flutter/example/run.sh to sanity check the shell syntax.

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• Public API changes reviewed by another Mobile SDK team member or implemented according to the develop docs spec
• No breaking changes

🔮 Next steps

If there's appetite for it, a follow-up could tighten test:dart/test:flutter/analyze:* to match the flags CI actually uses (randomized test ordering, web/wasm runs, --fatal-infos), since those currently drift from what CI checks.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.46%. Comparing base (130f1d1) to head (f53be24).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3945      +/-   ##
==========================================
+ Coverage   87.95%   92.46%   +4.51%     
==========================================
  Files         347      114     -233     
  Lines       12930     4391    -8539     
==========================================
- Hits        11372     4060    -7312     
+ Misses       1558      331    -1227     

Flag	Coverage Δ
sentry	?
sentry_dio	?
sentry_drift	?
sentry_file	?
sentry_firebase_remote_config	100.00% <ø> (ø)
sentry_flutter	92.17% <ø> (ø)
sentry_grpc	?
sentry_hive	?
sentry_isar	?
sentry_link	?
sentry_logging	?
sentry_sqflite	?
sentry_supabase	97.27% <ø> (ø)

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