Improve `load_accounts()` logging and defaults

Move `'paper'` default entry into the initial `bidict` instead of
appending post-loop. Add per-provider branch logging: an `info`-level
msg accumulating each loaded `account_alias` and a `debug`-level msg
(using `ppfmt()`) when a provider is skipped bc it wasn't requested.

Also,
- Early-`continue` when `accounts_section is None` instead of nesting
  inside an `else`.
- Import `ppfmt` from `tractor.devx.pformat`.
- Tighten union-type annotations to `X|Y` style.
- De-structure loop vars for readability.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/config.py

@@ -37,6 +37,7 @@ except ModuleNotFoundError:
 
 
 from tractor._exceptions import reg_err_types
+from tractor.devx.pformat import ppfmt
 from .log import get_logger
 
 log = get_logger('broker-config')
@@ -357,30 +358,56 @@ def write(
 
 
 def load_accounts(
-    providers: list[str] | None = None
-
-) -> bidict[str, str | None]:
+    providers: list[str]|None = None
+) -> bidict[str, str|None]:
 
     conf, path = load(
         conf_name='brokers',
     )
-    accounts = bidict()
-    for provider_name, section in conf.items():
-        accounts_section = section.get('accounts')
+    accounts = bidict({
+        # XXX, default paper-engine entry; this MUST be set.
+        'paper': None,
+    })
+    msg: str = (
+        'Loading account(s) from `brokers.toml`,\n'
+    )
+    for (
+        provider_name,
+        section,
+    ) in conf.items():
+        accounts_section: dict[str, str] = section.get('accounts')
+        if accounts_section is None:
+            msg += f'No accounts declared for {provider_name!r}?\n'
+            continue
+
+        # msg += f'Loaded accounts for {provider_name!r}?\n'
         if (
-            providers is None or
-            providers and provider_name in providers
+            providers is None
+            or (
+                providers
+                and
+                provider_name in providers
+            )
         ):
-            if accounts_section is None:
-                log.warning(f'No accounts named for {provider_name}?')
-                continue
-            else:
-                for label, value in accounts_section.items():
-                    accounts[
-                        f'{provider_name}.{label}'
-                    ] = value
+            for (
+                label,
+                value,
+            ) in accounts_section.items():
+                account_alias: str = f'{provider_name}.{label}'
+                accounts[account_alias] = value
+                msg += f'{account_alias} = {value!r}\n'
 
-    # our default paper engine entry
-    accounts['paper'] = None
+        else:
+            log.debug(
+                f'NOT loading account(s) for entry in `brokers.toml`,\n'
+                f'The account provider was not requested for loading.\n'
+                f'requested-providers: {providers!r}\n'
+                f'this-provider: {provider_name!r}\n'
+                f'\n'
+                f'{ppfmt(accounts_section)}\n'
+            )
+            # ?TODO? mk this bp work?
+            # breakpoint()
 
+    log.info(msg)
     return accounts