refactor(ironrdp-web): fix indexing_slicing clippy lint warnings

[RRRadicalEdward]

This lint

Checks for usage of indexing or slicing that may panic at runtime.

This one is quite pedantic and changes the way we write the code, but on the other hand, it makes the code more panic-resistant. Fixed the lint warnings only for ironrdp-web to see early feedback on it.
If we agree on adding it, I will add this lint for each crate separately, because there are too many warnings to do in 1 PR:

[github-actions]

Coverage Report 🤖 ⚙️

Past:
Total lines: 35869
Covered lines: 22616 (63.05%)

New:
Total lines: 35869
Covered lines: 22616 (63.05%)

Diff: +0.00%

[this comment will be updated automatically]

[RRRadicalEdward]

@CBenoit Does it look good? Should we proceed with adding this lint?

[CBenoit]

I want to double check a little bit more before merging this.

[CBenoit]

I’m putting back this PR on my TODO review list.

[RRRadicalEdward]

I’m putting back this PR on my TODO review list.

Hi @CBenoit. Did you have a chance to double-check this PR?

[CBenoit]

Sorry I actually forgot about it despite the TODO I wrote 😨

[RRRadicalEdward]

Sorry I actually forgot about it despite the TODO I wrote 😨

I just remembered about this PR yesterday. We have a mental connect 😄

[Copilot]

Pull request overview

This PR refactors the ironrdp-web crate to eliminate indexing_slicing clippy lint warnings by replacing direct slice indexing operations with safer alternatives that return Result or Option. The changes make the code more panic-resistant by ensuring all slice accesses are bounds-checked at runtime with proper error handling.

Changes:

• Converted slice indexing operations to use .get() and .get_mut() methods with appropriate error handling
• Updated function signatures to return Result types where slice access could fail
• Replaced array indexing in pixel processing with first_chunk() method

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
crates/ironrdp-web/src/image.rs	Modified extract_partial_image, extract_smallest_rectangle, and extract_whole_rows to return Result and use .get() for slice access with contextual error messages
crates/ironrdp-web/src/session.rs	Updated to propagate errors from extract_partial_image using the ? operator
crates/ironrdp-web/src/canvas.rs	Replaced direct indexing with first_chunk::<3>() for extracting RGB values from 4-byte pixel chunks
Cargo.toml	Added TODO comment noting intention to enable indexing_slicing lint project-wide

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