chore(gen): render OAuth/SCIM as initialisms, keeping o_auth as a deprecated alias

[rajarshidattapy]

Body

Fixes #587.

Problem

PR #561 normalized BYOK / STT / TTS via customCasings and left two initialisms behind.
The user-facing one is OAuth: the tag OAuth is mixed-case, so the snake_case attribute splits
on the internal capital and the namespace is client.o_auth, not client.oauth — while the
generated docs directory is docs/sdks/oauth/. Scim is the other: its class name never got
the initialism treatment that BYOK, STT, TTS, and APIKeys have.

Change

.speakeasy/gen.yaml                                      | +4   oauth + scim initialisms
.speakeasy/overlays/deprecated-o-auth-alias.overlay.yaml | new  o_auth kept as a deprecated alias
.speakeasy/workflow.yaml                                 | +1   overlay registered
.speakeasy/out.openapi.yaml                              | +5   overlay output kept in sync

Both entries use the same proven initialism: true form #561 introduced.

client.o_auth has been the only public name for these operations since v1.0, so the rename
is paired with a deprecated alias rather than shipped bare — a regen would otherwise break
every caller with no migration path. The overlay adds o_auth as a second tag on both OAuth
operations, mirroring deprecated-beta-responses-alias.overlay.yaml line for line, including
its note about defining the tag in the overlay so a monorepo sync can't silently drop it.

The two halves have very different blast radii

Worth knowing before merging, since initialism: true renders the token in all-caps
everywhere it appears — that is why #561 produced BYOKKey, STTRequest, STTInputAudio:

• oauth touches exactly one symbol. Grepping src/ for OAuth-containing symbols returns
  a single hit, the OAuth class. So this fixes the attribute and renames one class.
• scim touches six. Scim, plus ScimGroup, ScimGroupMapping, ScimGroupMappingRole,
  ScimGroupMappingTypedDict, ScimGroupTypedDict, and 3 docs pages.

The scim attribute is already correct, so unlike OAuth there is no ergonomics win — it is
cosmetic class-name consistency bought with six renamed public component classes. It is
included because #587 asks for it, but it is three lines to drop from gen.yaml and the OAuth
half stands alone. Happy to split it out if you'd prefer.

Verification

The Speakeasy CLI isn't available in my environment, so I verified the config and spec directly:

• customCasings is exactly {byok, oauth, scim, stt, tts}, all using initialism: true
• the new overlay is registered, and all 8 overlays exist and parse
• both overlay targets match in in.openapi.yaml (tags ['OAuth'], no pre-existing o_auth
  tag), so the overlay cannot silently no-op
• in out.openapi.yaml both operations now carry ['OAuth', 'o_auth'], the alias tag is
  present and deprecated: true with the sunset notice, the canonical OAuth tag is
  unchanged, and no other operation picked up the alias

src/ is intentionally unchanged

The rename only materializes on regeneration. Two things to confirm on the regen PR, since I
could not:

1. the attribute lands as client.oauth, with client.o_auth still present and marked deprecated
2. what the class renders as — the BYOK / STT precedent points to OAUTH, which is
   consistent with the other initialisms but is not the canonical spelling of OAuth. If you want
   OAuth preserved on the class while the attribute becomes oauth, that needs a different
   customCasings knob than the one chore(gen): render BYOK/TTS/STT as initialisms in generated symbols #561 established, and I could not find its schema in the
   public docs.