fix(react): #4515 inherit theme colour in slotted SVGs

[sylvesterkaczmarek]

Summary of the changes

Make React SlottedSVG icons inherit theme-aware component colours when no explicit fill is supplied.

• Default the SVG fill to currentColor.
• Preserve explicit consumer-provided fill values.
• Add a focused Cypress regression using the reported IcBreadcrumb pattern.
• Verify the icon's computed fill changes when the wrapping IcTheme changes from light to dark.

Related issue

Closes #4515

Validation

• Final branch diff audited against develop: two intended React files only.
• Focused Cypress regression coverage added.
• Local tests were not run because the repository dependency tree was unavailable in this workspace. GitHub Actions should validate the change after the PR is opened.
• No public API changes. Existing explicit SVG fill values remain supported.

Checklist

General

• All acceptance criteria reviewed and met.

Testing

• Relevant regression coverage added.
• GitHub Actions completed.

Accessibility

• Theme-aware icon colour behaviour covered.

Testing content extremes

• Explicit consumer-provided fill values remain supported.

[GCHQ-Developer-530]

I don't think this is quite working as expected because currentColor would make the svgs the link blue colour instead of black/white.

Also in order to get this running in storybook I had to change the Icon story for breadcrumb to use SlottedSVG, so while making the change to SlottedSVG is probably the correct thing to do, in order to test it you'll need to make those changes too on lines 164, 177, and 190 of ic-breadcrumb-group.stories.jsx in packages/react

[sylvesterkaczmarek]

Updated. SlottedSVG now defaults to the theme-aware neutral foreground token rather than currentColor, so breadcrumb icons use the appropriate light/dark foreground instead of inheriting the link colour.

I also updated the Breadcrumb Icon Storybook example to use SlottedSVG and tightened the Cypress regression to assert the resolved light and dark fill colours. The branch has also been rebuilt cleanly on current develop.

[GCHQ-Developer-299]

Tested locally with the updated story with/without the change to slottedSVG and confirmed that this works :)