feat!: declare MSRV 1.86 and enforce it in CI

[gagantrivedi]

Thanks for submitting a PR! Please check the boxes below:

• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

Contributes to #49

Follow-up to the reqwest 0.13 bump, which raised the crate's effective MSRV: a fresh cargo resolve pulls icu_* 2.2.0 and idna_adapter 1.2.2 (via url -> idna), which require rustc 1.86. Building with 1.85.0 fails out of the box:

error: rustc 1.85.0 is not supported by the following packages:
  icu_collections@2.2.0 requires rustc 1.86
  idna_adapter@1.2.2 requires rustc 1.86

This PR:

• Declares rust-version = "1.86.0" in Cargo.toml
• Adds an MSRV workflow running cargo +1.86.0 check, so future dependency bumps cannot silently invalidate the advertised MSRV (the existing Rust workflow only builds on latest stable)

The ! in the PR title is intentional: the breaking commit from #49 (feat!(deps): ...) could not be parsed by release-please (the ! must come after the scope: feat(deps)!:), so no release PR was created — see the workflow log:

commit could not be parsed: 68ad120 feat!(deps): bump reqwest to 0.13 (#49)
error message: Error: unexpected token '(' at 1:6, valid tokens [(, :]

Squash-merging this PR with the title as-is lets release-please pick up the breaking change and cut 3.0.0, covering both the reqwest 0.13 bump and the new MSRV.

How did you test this code?

• rustup toolchain install 1.86.0 --profile minimal and cargo +1.86.0 check on this branch with a fresh resolve — passes (Finished dev profile)
• cargo +1.85.0 check on a clean checkout of main — fails with the requires rustc 1.86 error above, confirming 1.86 is the true floor
• The new MSRV workflow runs on this PR itself, exercising the same check in CI