fix: immut over mut ref on validate

[rnbguy]

Closes: #864

Description

The core validate handler uses mutable reference, even though, the mutable reference is not needed.

ibc-rs/crates/ibc/src/core/handler.rs

Lines 54 to 58 in 133863d

	pub fn validate<Ctx>(
	ctx: &Ctx,
	router: &mut impl Router,
	msg: MsgEnvelope,
	) -> Result<(), RouterError>

This PR updates it to immutable reference.

ibc-rs/crates/ibc/src/core/handler.rs

Lines 54 to 58 in 99ca410

	pub fn validate<Ctx>(
	ctx: &Ctx,
	router: &impl Router,
	msg: MsgEnvelope,
	) -> Result<(), RouterError>

---

PR author checklist:

• Added changelog entry, using unclog.
• Added tests.
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (133863d) 71.49% compared to head (e59ada1) 71.48%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #863      +/-   ##
==========================================
- Coverage   71.49%   71.48%   -0.01%     
==========================================
  Files         125      125              
  Lines       15149    15145       -4     
==========================================
- Hits        10831    10827       -4     
  Misses       4318     4318              

Files Changed	Coverage Δ
crates/ibc/src/core/handler.rs	92.04% <100.00%> (-0.18%)	⬇️

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[plafer]

Can you add a changelog that refers to this PR?