fix: introduce AbsolutePathBuf and resolve relative paths in config.toml (#7796)

This PR attempts to solve two problems by introducing a
`AbsolutePathBuf` type with a special deserializer:

- `AbsolutePathBuf` attempts to be a generally useful abstraction, as it
ensures, by constructing, that it represents a value that is an
absolute, normalized path, which is a stronger guarantee than an
arbitrary `PathBuf`.
- Values in `config.toml` that can be either an absolute or relative
path should be resolved against the folder containing the `config.toml`
in the relative path case. This PR makes this easy to support: the main
cost is ensuring `AbsolutePathBufGuard` is used inside
`deserialize_config_toml_with_base()`.

While `AbsolutePathBufGuard` may seem slightly distasteful because it
relies on thread-local storage, this seems much cleaner to me than using
than my various experiments with
https://docs.rs/serde/latest/serde/de/trait.DeserializeSeed.html.
Further, since the `deserialize()` method from the `Deserialize` trait
is not async, we do not really have to worry about the deserialization
work being spread across multiple threads in a way that would interfere
with `AbsolutePathBufGuard`.

To start, this PR introduces the use of `AbsolutePathBuf` in
`OtelTlsConfig`. Note how this simplifies `otel_provider.rs` because it
no longer requires `settings.codex_home` to be threaded through.
Furthermore, this sets us up better for a world where multiple
`config.toml` files from different folders could be loaded and then
merged together, as the absolutifying of the paths must be done against
the correct parent folder.

codex-rs/Cargo.lock

@@ -1157,6 +1157,7 @@ dependencies = [
  "codex-otel",
  "codex-protocol",
  "codex-rmcp-client",
+ "codex-utils-absolute-path",
  "codex-utils-pty",
  "codex-utils-readiness",
  "codex-utils-string",
@@ -1464,6 +1465,7 @@ dependencies = [
  "chrono",
  "codex-app-server-protocol",
  "codex-protocol",
+ "codex-utils-absolute-path",
  "eventsource-stream",
  "http",
  "opentelemetry",
@@ -1650,6 +1652,16 @@ dependencies = [
  "codex-tui",
 ]
 
+[[package]]
+name = "codex-utils-absolute-path"
+version = "0.0.0"
+dependencies = [
+ "path-absolutize",
+ "serde",
+ "serde_json",
+ "tempfile",
+]
+
 [[package]]
 name = "codex-utils-cache"
 version = "0.0.0"