fix: remove footguns in IPC decoding and external string creation

[DonIsaac]

What does this PR do?

Follow-up to #17753

• fix(ipc): Handle the unlikely scenario that a message larger than webkit's size limit is received
• fix(string): strongly type context pointers when creating external strings
• docs(string): start documenting memory characteristics of string constructors. I'll be adding more over time
• refactor(string): remove RefString__free. It's only needed in one place and doesn't need to be a public API.

How did you verify your code works?

Existing tests should cover this.

[robobun]

^{Updated 3:12 PM PT - Feb 27th, 2025}

❌ @DonIsaac, your commit 2abb46d has 1 failures in Build #12434:

• test/js/bun/http/serve-body-leak.test.ts - 5 failing on 🐧 3.20 x64

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🧪   try this PR locally:

bunx bun-pr 17776