fix(compile): compile only for the requested script context

[vadim-anfv]

The policy is compiled for all three script contexts one after another, before --type is looked at, and any of those failing aborts the command. So a policy that is valid for the type you asked for is rejected because it is invalid for one of the other two.

Here a 9-of-16 multisig is compiled with --type tr and fails on the legacy context: CHECKMULTISIG takes at most 15 keys, and what the compiler falls back to goes past MAX_SCRIPT_ELEMENT_SIZE, the 520-byte consensus limit. Taproot has no such limit.

$ cargo run --all-features -- compile "thresh(9,pk(a),pk(b),pk(c),pk(d),pk(e),pk(f),pk(g),pk(h),pk(i),pk(j),pk(k),pk(l),pk(m),pk(n),pk(o),pk(p))" --type tr

thread 'main' panicked at miniscript-12.3.7/src/policy/compiler.rs:506:52:
Terminal creation must always succeed: ContextError(MaxRedeemScriptSizeExceeded)

The fix moves the compilation into the matching branch, so only the requested context is compiled.

Changelog notice

• Fixed compile rejecting policies that are valid for the requested script type

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.75%. Comparing base (e7223e5) to head (c626394).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
+ Coverage   57.45%   57.75%   +0.30%     
==========================================
  Files          22       22              
  Lines        3695     3686       -9     
==========================================
+ Hits         2123     2129       +6     
+ Misses       1572     1557      -15     

Flag	Coverage Δ
rust	57.75% <100.00%> (+0.30%)	⬆️

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