fix: Add is-relay field to pop build spec

[tsenovilla]

Follows:

• fix: Allow pop build spec to specify the runtime path as argument #687

This PR adds a new arg --is-relay to pop build spec, which may be used to avoid adding the para-id and relay keys to a relay chain spec

[Copilot]

Pull Request Overview

This PR introduces a --is-relay flag to the pop build spec command to support generating relay chain specifications without requiring parachain-specific fields (para-id and relay).

Key Changes:

• Added --is-relay argument that conflicts with para_id and relay arguments
• Modified BuildSpec struct to use Option<u32> for para_id and Option<RelayChain> for relay
• Updated logic to conditionally skip parachain-specific customizations when building relay chain specs

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[AlexD10S]

Nice work! Left a couple of small changes.

One more thing: can we cover the new --is-relay behavior in the integration tests?
For example, similar to the flow in https://github.com/r0gue-io/pop-cli/blob/main/crates/pop-cli/tests/chain.rs#L112 but using the --is-relay flag.

[moliholy]

LGTM! Just a small suggestion there, but feel free to discard it and merge.

[tsenovilla]

@AlexD10S @moliholy all comments addressed.

As discussed with Alex by DM, the integration test makes sense but we'd need to store/being able to generate with pop a relay chain to properly test it. It's not super worth it to do it now, but might be revisited in the future.

[AlexD10S]

LGTM!

[codecov]

Codecov Report

❌ Patch coverage is 90.10989% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.89%. Comparing base (3527bf0) to head (921384a).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/pop-cli/src/commands/build/spec.rs	90.10%	13 Missing and 5 partials ⚠️

@@           Coverage Diff           @@
##             main     #693   +/-   ##
=======================================
  Coverage   76.89%   76.89%           
=======================================
  Files         109      109           
  Lines       24549    24635   +86     
  Branches    24549    24635   +86     
=======================================
+ Hits        18877    18944   +67     
- Misses       3686     3702   +16     
- Partials     1986     1989    +3     

Files with missing lines	Coverage Δ
crates/pop-cli/src/commands/build/spec.rs	76.40% <90.10%> (+1.61%)	⬆️

... and 3 files with indirect coverage changes

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