feat: add TOML 1.1 support with multiline inline tables

[baszalmstra]

Description

Update TOML crates to support the TOML 1.1 specification, which introduces multiline inline tables and trailing commas. This improves readability of pixi.toml files by allowing users to write:

[dependencies]
python = {
    version = ">=3.12",
    channel = "conda-forge",
}

Crate updates (minimum versions for TOML 1.1):

• toml-span: 0.6.0 → 0.7.0 (TOML 1.1 parsing support)
• toml_edit: 0.23.0 → 0.25.0 (TOML 1.1 editing support)

Documentation updated to note TOML 1.1 support in the manifest reference.

How Has This Been Tested?

Added 6 new tests in pixi_manifest:

• 3 manifest parsing tests — verify multiline inline tables with trailing commas parse successfully for dependencies, nested build backend config, and mixed formatting
• 2 document editing tests (insta snapshots) — verify toml_edit preserves multiline inline table formatting when parsing and inserting values
• 1 error span test (insta snapshot) — verifies diagnostics point to the exact value inside a multiline inline table:

    8 │         bad-pkg = {
    9 │             version = "!invalid!",
      ·                        ─────────
   10 │             channel = "conda-forge",
  

All 285 pixi_manifest tests pass. Full project builds cleanly (cargo check succeeds).

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude Code

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.

[Hofer-Julian]

Really excited for this. I'd like to play a bit more with it locally before we merge it

[Hofer-Julian]

pixi add behaves a bit weird here:

[workspace]
authors = ["Julian Hofer <julianhofer@gnome.org>"]
channels = ["conda-forge"]
name = "toml-1.1"
platforms = ["linux-64"]
version = "0.1.0"

[tasks]

[feature.test]
dependencies = {
    numpy = "*"
}

[environments]
test = ["test"]

Then run pixi add --feature test pydantic gives me this:

[feature.test]
dependencies = {
    numpy = "*"
, pydantic = ">=2.12.5,<3" }

I would have expected:

[feature.test]
dependencies = {
    numpy = "*",
    pydantic = ">=2.12.5,<3", 
}

[baszalmstra]

I converted to draft because I think this could be integrated better in more places. Im working on that now.